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

Issue 1: NACK packets may contain sequence numbers that are too old to be in the receiver's SentPacketHistory.

Fix: Before sending a NACK, the missing set of the relevant SequenceNumberStats is pruned (using the same old-age cutoff as the number of packets kept in SentPacketHistory).  This way, no super-old sequence numbers will be packed into the NACK packet

Test: not tested


Issue 2: AudioRingBuffer length was hardcoded for all instances at 10.  This may be undesirable since upstream and downstream audio may have different amounts of jitter,

Fix: The constructors of AudioRingBuffer and its subclasses now have an additional arg numFramesCapacity.   AudioMixerClientData.cpp (upstream) now makes all _ringBuffers to have 100 frames while Audio.cpp (downstream) keeps its ring buffer at 10 frames.

Test: not tested
Make the numFramesCapacity be the last parameter and give it a default value so that you don't have to change all the callers. In general when you add new functionality like this, that will only be used in some cases, you should add it in a manner that's transparent to all the old users, so you don't have to change the code everywhere.