This fixes a race condition: Peer sends updated SDP with new address,
but an older RTP packet from the old address is received afterwards.
Thsi triggers learning of this old address is the "correct" endpoint.
Afterwards the peer stops sending RTP until a packet to the new endpoint
is received there, which never happens because the new endpoint has been
discarded in favour of the "learned" old one.
closes#817
Change-Id: I508f465a669f03e35ddcc6e770d5e7859e57569f
(cherry picked from commit 45bd10d323)