Improvement in packet lost concealment (PLC) when handling burst of lost packets

[bennylp]

Initial commit in r2850:

WSOLA improvements:

• Introduce fade-out and fade-in effect
• Limit the number of continuous synthetic samples (only take effect when fading is used)
• Export many settings as macros:
  • PJMEDIA_WSOLA_DELAY_MSEC (was HANNING_PTIME)
  • PJMEDIA_WSOLA_TEMPLATE_LENGTH_MSEC (was TEMPLATE_PTIME)
  • PJMEDIA_WSOLA_MAX_EXPAND_MSEC

PLC:

• added compile time macro PJMEDIA_WSOLA_PLC_NO_FADING to disable fading (default enabled)

Stream:

• fixed bug when stream is not PLC-ing subsequent packet loss (only the first)
• also add maximum PLC limit just as precaution if PLC doesn't limit number of synthetic frames
• unrelated: fixed warning about unused send_keep_alive() function

Delaybuf:

• modified to NOT use fading in WSOLA since we don't expect it to generate many continuous synthetic frames