Added timeout to WaitAsync method

[nzaugg]

Renamed the _mutex variable to _syncRoot, as it's actually a sync root for a monitor lock and is not an actual mutex

[StephenCleary]

Hello, nzaugg,

Thank you for the pull request. However, not having int/TimeSpan timeout overloads is a deliberate decision. The AsyncEx way of doing timeouts is to use CancellationTokens.

On a side note, your implementation would use a single timer for the entire AsyncManualResetEvent instance, which would have incorrect semantics. You'd need a timer per method call so that the timer isn't shared.