fix: honor timeouts for long sync check

[t3hmrman]

Resolves #4

As of PR creation, the new check passes and none of the others do:

cargo nextest run -E 'test(test_unit_sync_executor_with_long_check)'

A couple things left:

• [x] Investigate using RwLock instead of Mutex
• [x] Finish implementing the cases in the check loop

[t3hmrman]

Since checking actually attempts to take the value (and exit early), Mutex actually makes the most sense (there is no read-only activity).