fix(pageserver): consider partial compaction layer map in layer check

[skyzh]

Problem

In #9897 we temporarily disabled the layer valid check because the current one only considers the end result of all compaction algorithms, but partial gc-compaction would temporarily produce an "invalid" layer map.

part of #9114

Summary of changes

Allow LSN splits to overlap in the slow path check. Currently, the valid check is only used in storage scrubber (background job) and during gc-compaction (without taking layer lock). Therefore, it's fine for such checks to be a little bit inefficient but more accurate.

[github-actions]

7095 tests run: 6798 passed, 0 failed, 297 skipped (full report)

---

Flaky tests (1)

Postgres 16

• test_physical_replication_config_mismatch_max_locks_per_transaction: release-x86-64

Code coverage* (full report)

• functions: 31.3% (8398 of 26866 functions)
• lines: 48.0% (66671 of 138978 lines)

* collected from Rust tests only

---

_{The comment gets automatically updated with the latest test results
90c5649 at 2024-12-18T22:21:37.718Z :recycle:}

[problame]

Can we run this check also at Step 3, while we hold the layer map lock, after updating layer map in finish_gc_compaction?

(And ideally roll back the change, or panic the tenant?)

[skyzh]

a16a91f adds a final check for the layer map before updating the compaction result back to the layer map.