fix: splitstore: Don't deadlock in mpool protector

[magik6k]

Related Issues

Addresses #9846

Proposed Changes

Don't deadlock in splitstore compaction when applying mpool protector; Bail from compaction when mpool is busy.

A better fix would be to maybe drop the splitstore lock while applying protectors, and re-get the lock in protector callback.

Additional Info

My node has hit this 4 times in a row now, and it's very annoying, so I have fixed it.

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[arajasek]

Acceptable quickfix, I think.