Fixes deadlock due to mutex double lock.

This will be used by filters which have internal buffers which need to
be flushed at the end, e.g. the "soxr" resampler.

This call cannot fail, but needs cleanup if others fail.  By moving it
to the end, we eliminate some cleanup.

Moved code from AudioOutputControl::InternalOpen2().

struct AudioOutput should not know or care about multi-threading.

Prepare for moving the AudioOutputSource to class AudioOutputControl.

That function is "noexcept".