WIP: Fix rtpjitterbuffer packet spacing calculations [ci skip]

Note that the unit tests haven't yet been updated accordingly and therefore will fail. I just wanted to get some preliminary feedback first, and hopefully have it tested in more real-life scenarios.

rtpjitterbuffer: Statistical packet spacing calculation

Instead of the previous heuristic calculation, calculate the average and
standard deviation of a moving window of 50 samples, and then use μ +
1.5 * σ as a timeout.

rtpjitterbuffer: Fix expected arrival time of next seqnum

We should expect the next seqnum to arrive after a whole packet spacing.
The previous calculation was using half the packet spacing, which
doesn't go far enough.