Add `KeyedMutex`

[BalmungSan]

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Extention to the current Mutex class to support one look per key.

I extracted the LockQueue and lock implementations into their own file to allow reuse.

I added a basic suite of tests, mostly focused on multiple key interactions. Since each key is, in essence, its own Mutex I don't think we need to replicate all the complex Mutex tests... since those are actually tests for the LockQueue.