This is the async-ready equivalent of ManualResetEvent, similar to Stephen Toub's AsyncManualResetEvent.
Like other "events", an AsyncManualResetEvent is either set or unset at any time. An AsyncManualResetEvent can be changed from unset to set by calling its Set method, and it can be changed from set to unset by calling its Reset method.
When an AsyncManualResetEvent is in the set state, it will satisfy all waiters. Calling Set or Reset when the AsyncManualResetEvent is already in that state is a noop.
The task returned from WaitAsync will enter the Completed state when the AsyncManualResetEvent is in the set state.
AsyncManualResetEvent also supports synchronous waiting with the Wait method.
You can call Wait with an already-cancelled CancellationToken to test whether the AsyncManualResetEvent is in the set state.