Change jitter buffer progressive discard algorithm

[nanang]

Issues with current algorithm:

• Precision issue in discard rate calculation in JB progressive discard. When jb-size/delay equal to burst-level, jb frame discard should not occur (delay in JB is expected to be always equal or less than burst-level). But with current calculation, the JB will still discard some frames because of bad precision in discard rate calculation.
• The discard rates were predefined and hardcoded based on excessive latency, this may not suit well for some environments. So, there is an idea to calculate discard rate dynamically based on parameters such as actual burst level, excessive latency, aggressiveness setting.

[nanang]

(In [3779]) Fix #1378: replaced discard rate decision based on overflow precentage (precentage of excess delay compared to burst-level).

[nanang]

(In [3814]) Re #1378:

• Implemented new algorithm for JB progressive discard.
• Added new API and for setting JB discard algorithm at run-time.
• Updated JB test for the new algorithm.