XAudio2 DestroyVoice call causes significant hitching on some architectures

Another thread:

https://answers.unrealengine.com/questions/241358/amd-cpu-very-low-performance-because-ixaudio2sourc.html

Note: This not a regression and has existed since our first implementation of XAudio2 in UE4.

The ideal way to fix this would be to have the entire audio engine on a separate thread (which the new audio engine will be), but for now we can make an async task to call DestroyVoice on the IXAudio2SourceVoices without too much issue.