Add AsyncMutex class

[AArnott]

This adds a more convenient way to execute async code while holding (or waiting for) a mutex.

Mutexes are traditionally very difficult to asynchronously wait for or do async work while holding because they are thread affinitized. This new class creates a dedicated thread to 'own' the mutex while allowing the AsyncMutex owner to await for the mutex and run async code on any thread while holding it.

[AArnott]

The linux failure is when the net472 build of the library is executed on the mono runtime. I've been dropping support for mono lately and maybe it's time to drop it from this library as well. But I'm trying a couple things to see if I can get it to work for the new code first.

[lifengl]

looks good to me if this is intended for a narrow usage scenario. It is somewhat different from the Mutex, so I am not sure whether using a generic name like an async version of the original class is a good idea, as it may confuse some users.