Purge old FEC packets from receiver's queue before media sequence numbers wrap around

- Received FEC packets are purged from the queue if:
  1. All media packets protected by the FEC packet are received.
  2. All media packets protected by the FEC packet are recovered.
  3. Newer FEC packet(s) with sequence number '0x3fff' larger than an old FEC packet is received.
- When FEC packets get separated from their protected media packets by more than 48, none of the first conditions ever delete that FEC packet, no matter how old/ irrelevant it gets.
- Under specific circumstances, the new FEC packet (condition 3) is not received before the media sequence number space wraps around, and incorrectly activates the old FEC packet, resulting in FEC decode for the wrong packet.
- This change purges such old FEC packets in time before the media sequence numbers wrap around.

Bug: webrtc:12656
Change-Id: I6ddf5382638c8c7e9a65724b2544dfbbc4803342
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/215100
Reviewed-by: Rasmus Brandt <brandtr@webrtc.org>
Reviewed-by: Åsa Persson <asapersson@webrtc.org>
Commit-Queue: Rasmus Brandt <brandtr@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#33795}