Merge GetRTP and Nack into GetPacket.

The two function were always called together.  This factors out
the NACKing logic into the track.

conn/conn.go

@@ -25,9 +25,9 @@ type UpTrack interface {
 	Kind() webrtc.RTPCodecType
 	Label() string
 	Codec() webrtc.RTPCodecCapability
-	// get a recent packet.  Returns 0 if the packet is not in cache.
-	GetRTP(seqno uint16, result []byte) uint16
-	Nack(seqnos []uint16) error
+	// GetPacket fetches a recent packet.  Returns 0 if the packet is
+	// not in cache, and, in that case, optionally schedules a NACK.
+	GetPacket(seqno uint16, result []byte, nack bool) uint16
 	RequestKeyframe() error
 }