rtpjitterbuffer: clean up and improve missing packets handling

• Try to make variable and function names more clear.

• Add plenty of comments describing the logic step-by-step.

• Improve the logging around this, making the logs easier to read and understand when debugging these issues.

• Revise the logic of packets that are actually beyond saving in doing the following:

1. Do an optimistic estimation of which packets can still arrive.
2. Based on this, find which packets (and duration) are now hopelessly lost.
3. Issue an immediate lost-event for the hopelessly lost and then add lost/rtx timers for the ones we still hope to save, meaning that if they are to arrive, they will not be discarded.

• Revise the use of rtx-delay: Earlier the rtx-delay would vary, depending on the pts of the latest packet and the estimated pts of the packet it being issued a RTX for, but now that we aim to estimate the PTS of the missing packet accurately, the RTX delay should remain the same for all packets. Meaning: If the packet have a PTS of X, the delay in asked for a RTX for this packet is always a constant X + delay, not a variable one.

• Finally ensure that the chaotic "check-for-stall" tests uses timestamps that starts from 0 to make them easier to debug.