Implement renegotiation of down streams.

We used to destroy and recreate down streams whenever something changed,
which turned out to be racy.  We now properly implement renegotiation,
as well as atomic replacement of a stream by another one.

conn/conn.go

@@ -17,7 +17,6 @@ type Up interface {
 	DelLocal(Down) bool
 	Id() string
 	User() (string, string)
-	Codecs() []webrtc.RTPCodecCapability
 }
 
 // Type UpTrack represents a track in the client to server direction.