freezeblocks: fix max visible segment when there is a gap in .idx files

[taratorio]

We have a gap in our bor mainnet snapshot toml between
'v1-051900-052000-headers.idx' = 'd97b3c598f520888eb67244ba28d994998584e65'
'v1-054000-054100-headers.idx' = 'a20efd5aca03ebb528215b24ce6aa0c20cc90c55'

When syncing bor mainnet from scratch the node fails to sync due to that. We will fix the gap in our toml however the node should auto-recover by regenerating the .idx files

After digging further it seems like we have a regression in #11557
idxAvailability used to stop when finding a seg which is not indexed
after that PR it does not, but instead it keeps on going so idxMax gets set to 62 mil although there is a gap (it should have been set to 52 mil where the gap is)

Tested on my node and it recreated the gap of indices

[INFO] [10-09|16:32:27.574] [snapshots:missed-idx] Stat              blocks=62.44M indices=52.00M alloc=26.8GB sys=27.4GB
[DBUG] [10-09|16:32:27.957] [index] created                          file=/home/admin/erigon-data/e3-bor-heimdall-stage-mainnet/snapshots/v1-052500-052600-headers.idx.tmp
[DBUG] [10-09|16:32:27.957] [index] calculating                      file=v1-052500-052600-headers.idx
[DBUG] [10-09|16:32:27.977] [index] created                          file=/home/admin/erigon-data/e3-bor-heimdall-stage-mainnet/snapshots/v1-053200-053300-headers.idx.tmp
[DBUG] [10-09|16:32:27.977] [index] calculating                      file=v1-053200-053300-headers.idx
[DBUG] [10-09|16:32:27.978] [index] created                          file=/home/admin/erigon-data/e3-bor-heimdall-stage-mainnet/snapshots/v1-052800-052900-headers.idx.tmp
[DBUG] [10-09|16:32:27.978] [index] calculating                      file=v1-052800-052900-headers.idx
[DBUG] [10-09|16:32:27.978] [index] created                          file=/home/admin/erigon-data/e3-bor-heimdall-stage-mainnet/snapshots/v1-053000-053100-headers.idx.tmp
[DBUG] [10-09|16:32:27.978] [index] calculating                      file=v1-053000-053100-headers.idx
[DBUG] [10-09|16:32:27.980] [index] created                          file=/home/admin/erigon-data/e3-bor-heimdall-stage-mainnet/snapshots/v1-052300-052400-headers.idx.tmp
[DBUG] [10-09|16:32:27.980] [index] calculating                      file=v1-052300-052400-headers.idx
[DBUG] [10-09|16:32:27.983] [index] created                          file=/home/admin/erigon-data/e3-bor-heimdall-stage-mainnet/snapshots/v1-052700-052800-headers.idx.tmp
[DBUG] [10-09|16:32:27.983] [index] calculating                      file=v1-052700-052800-headers.idx
[DBUG] [10-09|16:32:27.990] [index] created                          file=/home/admin/erigon-data/e3-bor-heimdall-stage-mainnet/snapshots/v1-052400-052500-headers.idx.tmp
[DBUG] [10-09|16:32:27.990] [index] calculating                      file=v1-052400-052500-headers.idx
[DBUG] [10-09|16:32:27.994] [index] created                          file=/home/admin/erigon-data/e3-bor-heimdall-stage-mainnet/snapshots/v1-053300-053400-headers.idx.tmp
[DBUG] [10-09|16:32:27.994] [index] calculating                      file=v1-053300-053400-headers.idx
[DBUG] [10-09|16:32:27.996] [index] created                          file=/home/admin/erigon-data/e3-bor-heimdall-stage-mainnet/snapshots/v1-052200-052300-headers.idx.tmp
[DBUG] [10-09|16:32:27.996] [index] calculating                      file=v1-052200-052300-headers.idx
[DBUG] [10-09|16:32:27.997] [index] created                          file=/home/admin/erigon-data/e3-bor-heimdall-stage-mainnet/snapshots/v1-052600-052700-headers.idx.tmp
[DBUG] [10-09|16:32:27.997] [index] calculating                      file=v1-052600-052700-headers.idx
[DBUG] [10-09|16:32:28.001] [index] created                          file=/home/admin/erigon-data/e3-bor-heimdall-stage-mainnet/snapshots/v1-052100-052200-headers.idx.tmp
[DBUG] [10-09|16:32:28.001] [index] calculating                      file=v1-052100-052200-headers.idx
[DBUG] [10-09|16:32:28.002] [index] created                          file=/home/admin/erigon-data/e3-bor-heimdall-stage-mainnet/snapshots/v1-052000-052100-headers.idx.tmp
[DBUG] [10-09|16:32:28.002] [index] calculating                      file=v1-052000-052100-headers.idx
[DBUG] [10-09|16:32:28.016] [index] created                          file=/home/admin/erigon-data/e3-bor-heimdall-stage-mainnet/snapshots/v1-053100-053200-headers.idx.tmp
[DBUG] [10-09|16:32:28.016] [index] calculating                      file=v1-053100-053200-headers.idx
[DBUG] [10-09|16:32:28.020] [index] created                          file=/home/admin/erigon-data/e3-bor-heimdall-stage-mainnet/snapshots/v1-052900-053000-headers.idx.tmp
[DBUG] [10-09|16:32:28.020] [index] calculating                      file=v1-052900-053000-headers.idx
[DBUG] [10-09|16:32:28.023] [index] created                          file=/home/admin/erigon-data/e3-bor-heimdall-stage-mainnet/snapshots/v1-053400-053500-headers.idx.tmp
[DBUG] [10-09|16:32:28.023] [index] calculating                      file=v1-053400-053500-headers.idx
[DBUG] [10-09|16:32:28.235] [index] write                            file=v1-053200-053300-headers.idx
[DBUG] [10-09|16:32:28.237] [index] write                            file=v1-052800-052900-headers.idx
[DBUG] [10-09|16:32:28.237] [index] write                            file=v1-052700-052800-headers.idx
[DBUG] [10-09|16:32:28.244] [index] write                            file=v1-052400-052500-headers.idx
[DBUG] [10-09|16:32:28.244] [index] write                            file=v1-052100-052200-headers.idx
[DBUG] [10-09|16:32:28.265] [index] write                            file=v1-053000-053100-headers.idx
[DBUG] [10-09|16:32:28.268] [index] write                            file=v1-052300-052400-headers.idx
[DBUG] [10-09|16:32:28.270] [index] write                            file=v1-052500-052600-headers.idx
[DBUG] [10-09|16:32:28.286] [index] write                            file=v1-052900-053000-headers.idx
[DBUG] [10-09|16:32:28.288] [index] write                            file=v1-052600-052700-headers.idx
[DBUG] [10-09|16:32:28.292] [index] write                            file=v1-052000-052100-headers.idx
[DBUG] [10-09|16:32:28.296] [index] write                            file=v1-053100-053200-headers.idx
[DBUG] [10-09|16:32:28.319] [index] write                            file=v1-052200-052300-headers.idx
[DBUG] [10-09|16:32:28.322] [index] write                            file=v1-053300-053400-headers.idx
[DBUG] [10-09|16:32:28.339] [index] write                            file=v1-053400-053500-headers.idx
[DBUG] [10-09|16:32:28.508] [index] created                          file=/home/admin/erigon-data/e3-bor-heimdall-stage-mainnet/snapshots/v1-053800-053900-headers.idx.tmp
[DBUG] [10-09|16:32:28.508] [index] calculating                      file=v1-053800-053900-headers.idx
[DBUG] [10-09|16:32:28.520] [index] created                          file=/home/admin/erigon-data/e3-bor-heimdall-stage-mainnet/snapshots/v1-053900-054000-headers.idx.tmp
[DBUG] [10-09|16:32:28.520] [index] calculating                      file=v1-053900-054000-headers.idx
[DBUG] [10-09|16:32:28.521] [index] created                          file=/home/admin/erigon-data/e3-bor-heimdall-stage-mainnet/snapshots/v1-053600-053700-headers.idx.tmp
[DBUG] [10-09|16:32:28.521] [index] calculating                      file=v1-053600-053700-headers.idx
[DBUG] [10-09|16:32:28.526] [index] created                          file=/home/admin/erigon-data/e3-bor-heimdall-stage-mainnet/snapshots/v1-053700-053800-headers.idx.tmp
[DBUG] [10-09|16:32:28.526] [index] calculating                      file=v1-053700-053800-headers.idx
[DBUG] [10-09|16:32:28.529] [index] created                          file=/home/admin/erigon-data/e3-bor-heimdall-stage-mainnet/snapshots/v1-053500-053600-headers.idx.tmp
[DBUG] [10-09|16:32:28.529] [index] calculating                      file=v1-053500-053600-headers.idx
[DBUG] [10-09|16:32:28.756] [index] write                            file=v1-053800-053900-headers.idx
[DBUG] [10-09|16:32:28.780] [index] write                            file=v1-053900-054000-headers.idx
[DBUG] [10-09|16:32:28.782] [index] write                            file=v1-053600-053700-headers.idx
[DBUG] [10-09|16:32:28.783] [index] write                            file=v1-053700-053800-headers.idx
[DBUG] [10-09|16:32:28.805] [index] write                            file=v1-053500-053600-headers.idx
[INFO] [10-09|16:32:30.944] [snapshots:missed-idx:reopen] Stat       blocks=62.44M indices=62.44M alloc=21.7GB sys=32.5GB