Simplify the protocol and the protocol interface.

Split the id field into id and source, where source indicates the sender
of the message and id the entity being sent.  Remove the label request,
just use the offerer's username.  Maintain the username within the
ServerConnection, this removes a parameter from some methods.

conn/conn.go

@@ -16,7 +16,7 @@ type Up interface {
 	AddLocal(Down) error
 	DelLocal(Down) bool
 	Id() string
-	Label() string
+	User() (string, string)
 	Codecs() []webrtc.RTPCodecCapability
 }