feat: fix deadlock in splitstore-mpool interaction

[arajasek]

Related Issues

Fixes #9846.

This deadlock was being caused by attempting to access the warmupepoch variable, which was behind the splitstore.mx lock. We can just access this variable atomically.

Proposed Changes

• Access warmupEpoch atomically
• Revert fix: splitstore: Don't deadlock in mpool protector #9903: The deadlock reported in bug: splitstore: deadlock during compaction (setup?) #9846 should be fixed by this PR. We might emerge other deadlocks by reverting 9903, but we really shouldn't be locking the entire splitstore to access -- any other deadlocks that emerge can be considered bugs we fix.

Additional Info

• Supersedes feat: deflake splitstore_test #10831, which should no longer be needed if we land this

Checklist

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[magik6k]

1 comment, other than that looks good

[magik6k]

Can't tell if this int is properly aligned, probably use the atomic.Int64 type, which is way safer/harder to mess up.

[ZenGround0]

Can also remove lock around warmupEpoch here

Maybe mx should now be renamed protectorsLk since its only protecting protectors now afaict.