RTP: compute deadline from RX time of oldest packet in queue

RTP: compute deadline from RX time of oldest packet in queue

Jitter computation is, by design, relative to the time of arrival of
packets. Normally, it should be the estimated time of arrival (ETA) of
the missing packet in the sequence, plus a tolerance value computed
from the delay variance (jitter). Unfortunately, we do not know that
ETA since we do not know the RTP timestamp of missing packets.