Improve package loss concealment

[doe300]

Currently, we replace lost packages with silent-packages.

• This is the simplest, but also the worst technique, which results in cut off speech when several packages are lost.
• A possibly better alternative would be to repeat the last correctly received package. Since the contents of two successive packages are often similar, this will result in much better audio-output when losing packages.
• Further improve concealment by adjusting the thresholds (silence, number of copied packages) and replacing silence with comfort noise.

See:
https://en.wikipedia.org/wiki/Packet_loss_concealment
http://flylib.com/books/en/4.245.1.63/1/

[doe300]

As of 752b713, RTPBuffer repeats the last successfully received package X (X = 3) times and then conceals successive losses with silence packages