chore: loadShards changes to more cleanly support 2.x feature (#25513)

* chore: move shardID parsing and shard filtering into walkShardsAndProcess

* chore: make it impossible to miss sending shardResponse or marking shard as complete

* chore: always count number of shards (preparation for 2.x related feature)

* chore: explicitly load series files and create indices serially

Explicitly load series files and create indices serially. Also
avoid passing them to work functions that don't need them.

* chore: rework loadShards for changes necessary to cancel loading process

* chore: comment improvements

* fix: fix race conditions in TestStore_StartupShardProgress and TestStore_BadShardLoading

* chore: avoid logging nil error

* chore: refactor shard loading and shard walking

Refactor loadShards and CreateShard to use a common shardLoader class that
makes thread-safety easier. Refactor walkShardsAndProcess into findShards.

* chore: improve comment

* chore: rename OpenShard to ReopenShard and implement with shardLoader

Rename Store.OpenShard to Store.ReopenShard and implement using a
shardLoader object. Changes to tests as necessary.

* chore: avoid resetting shard options and locking on Reopen

Avoid resetting shard options when reopening a shard.
Proper mutex locker in Shard.ReopenShard.

* chore: fix formatting issue

* chore: warn on mixed index types in Store.CreateShard

* chore: change from info to warn when invalid shard IDs found in path

* chore: use coarser locking in Store.ReopenShard

* chore: fix typo in comment

* chore: code simplification