Fix mutex cancelled acquire

[durban]

Fix Async mutex acquire to be cancellation-safe.

[armanbilge]

Unfortunately this doesn't seem to have fixed the Skunk issue 😕 tried it in armanbilge/skunk@7a2ab6a.

[armanbilge]

The test you added is good but unfortunately it's affected by #3503 😕

[armanbilge]

Is this cancelation safe? Can the async which created this waiter be canceled before it is invoked?

cats-effect/std/shared/src/main/scala/cats/effect/std/Mutex.scala

Lines 127 to 129 in c760a55

	var waiter = waiters.take()
	while (waiter eq null) waiter = waiters.take()
	waiter(RightTrue) // pass the buck

[durban]

Yeah, I think that's the same problem as the one was with Queue. Probably could be fixed the same way...

[armanbilge]

Oh right. How was that one fixed? 😆