Fix inconsistent synchronization of PoolSubpage.doNotDestroy #12775
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Motivation:
This field is modified under lock, and thus should not be accessed (toString excepted) without locking.
Modification:
Move an assert check on PoolSubpage.doNotDestroy into a nearby critical region.
Also encapsulate the findSubpagePoolHead logic better; PoolArena now always use this method instead of that one place that was accessing the array directly. This should not have any influence on the logic.
Result:
Assert in PoolChunk is no longer racy.