Maintain a reference to the connection associated to each track.

This avoids carrying an extra parameter in many places.

conn/conn.go

@@ -27,7 +27,7 @@ type UpTrack interface {
 	Codec() webrtc.RTPCodecCapability
 	// get a recent packet.  Returns 0 if the packet is not in cache.
 	GetRTP(seqno uint16, result []byte) uint16
-	Nack(conn Up, seqnos []uint16) error
+	Nack(seqnos []uint16) error
 	RequestKeyframe() error
 }