[core] Added jitter tracing

[maxsharabayko]

Adding RFC3550-like jitter metrics for:

• jitter of incoming data packets over UDP on the receiving side;
• jitter of sending packets over UDP on the sending side;
• jitter of reading packets from SRT by the receiving app.

TODO

• Statistics API is affected (more fields in SRT_BSTATS). Consider API/ABI compatibility.
• Document how jitter is calculated.

[BlueWaterCrystal]

Is it currently possible to test the PR by itself?
bstats jitter seems to return 0 On receiver and sender jitter.

[maxsharabayko]

@BlueWaterCrystal
Thanks for reporting. Forgot to copy internal jitter values to the SRT stats API. :)
It should work now (have not checked myself yet).