prune missing set before constructing nack packets;

make MAX_REASONALBE_SEQUENCE_GAP global and sentPacketHistory to use
that value as the default history size
nacked edit packets are not resent if NACKs are disabled in menu
made AudioRingBuffer frames capacity variable
switched order of args in PositionalAudioRingBuffer
fixed bug in SequenceNumberStats and its unit test; test passes now
moved and updated a comment in SequenceNumberStats
removed numFramesCapacity from being exposed everywhere
forgot arg in AudioRingBuffer constructor in PositionalAudioRingBUffer

changed _currentJitterBufferFrames to be -1 to indicate starved.
fixed more small bugs with AudioRingBuffer constructor
Merge pull request #3107 from wangyix/master

prune missing sequence numbers before sending NACK; AudioRingBuffer frame length can now be specified;