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
The NaN is probably getting injected into nextLoudness above when _numFrameSamples happens to be zero.  Thereafter any math done with nextLoudness will produce NaN.  Rather than check for NaN at the end you could just exit early when samplesAvailable() is zero.  This is just FYI and doesn't block the merge.