rtpjitterbuffer: late packets shouldn't affect PTS of the following packet

If, say, a rtx-packet arrives really late, this can have a dramatic
effect on the jitterbuffer clock-skew logic, having it being reset
and losing track of the current dts-to-pts calculations, directly affecting
the packets that arrive later.

This is demonstrated in the test, where a RTX packet is pushed in really
late, and without this patch the last packet will have its PTS affected
by this, where as a late RTX packet should be redundant information, and
not affect anything.
18 jobs for pexip-jb-patches in 123 minutes and 58 seconds (queued for 104 minutes and 13 seconds)
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