Made audio timegap stats more readable with varying units; Fixed AudioMixer NaN error on WIndows;

Issue 1: audio packets timegap stats displayed in interface were always in usecs. 

Fix: Implemented a function to format a usec value to display as us, ms, s, or min.

Testing: ran interface and verified the new stats format worked


Issue 2: PositionalAudioRingBuffer::updateNextOutputTrailingLoudness() sometimes calculates _nextOutputTrailingLoudness to be NaN.  Don't know what causes this, but seems to be Windows-related.  This NaN value for loudness can cause a later minimum loudness comparison to fail, which will incorrectly prevent a stream from being mixed in the AudioMixer.

Fix: added a isNan() test to updateNextOutputTrailingLoudness().  If the new value is NaN, _nextOutputTrailingLoudness is reverted to its old value.

Testing: Mic audio no longer cuts out randomly on my Windows machine (running local domain server and AudioMixer).



Other things fixed: made some string buffers used for storing formatted stats strings in Stats.cpp larger to prevent overflow; fixed warning about initialization order in Audio.cpp