refactor(send queue): use a specialized mutex for locking access to the state store and `being_sent`

[bnjbvr]

There was an implicit relationship that the being_sent lock needed to be taken in order to do non-atomic state store operations. With the change from this commit, the relationship is now more explicit: to get a handle to the state store, or being_sent, you have to obtain a StoreLockGuard by locking against the store itself. The WeakClient isn't stored in the QueueStorage data structure itself, so it's the only way to get a dyn StateStore from the QueueStorage.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 85.02%. Comparing base (a8a83c3) to head (01ed8ad). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #4268 +/- ## ========================================== - Coverage 85.03% 85.02% -0.01% ========================================== Files 274 274 Lines 30028 30039 +11 ========================================== + Hits 25533 25541 +8 - Misses 4495 4498 +3 ```

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