feat: remove the last bits of arc-swap

[demosdemon]

I noticed there was one remaining component using arc-swap. This change replaces it with a standard Mutex. And, removes the dependency on arc-swap.

[AminR443]

Sounds great. I planned on doing this sometime soon, was not sure about performance implications though; Are we sure that mutex is the best option here too?

[demosdemon]

Are we sure that mutex is the best option here too?

The parent here has nearly the same usages as MaybePersisted; it is only accessed during commit -- to verify the parent is committed and has the expected hash (parent_hash_is) or to indicate the parent is committed and set its hash (commit_reparent). There should be no contention on the lock right now since there's no parallelism on commits.