RTCP statistics updates (thanks Roman Puls for the suggestions)

[nanang]

Updates suggested:

1. Currently the RFC 3550 compliant jitter calculation is used in generating RTCP RR packet, however the jitter statistics is calculated from "raw jitter" (referring to "different D" of RFC 3550), so changes to be done:
   • jitter statistics should use RFC 3550
   • add "raw jitter" statistics (calculated from absoluted RFC 3550 diff D), compile-time configurable, disabled by default.
   • also add IPDV (calculated from RFC 3550 diff D), compile-time configurable, disabled by default.
2. Add feature to reset RTCP statistics.

[nanang]

(In [3237]) Re #1103:

• Updated RTCP jitter statistics calculation (in receiving direction) to use "interarrival jitter" (was using "difference D") of RFC 3550.
• Added APIs to reset RTCP statistics.

[nanang]

(In [3239]) Re #1103:

• Added (back) raw jitter statistics into RTCP statistics, with the new name "rx_raw_jitter".
• Added IPDV statistics into RTCP statistics.
• Added new compile-time settings to enable/disable raw jitter and IPDV statistics.
• Updated call dump in pjsua-lib.