p2p: msg.Payload contains list data

With RLPx frames, the message code is contained in the
frame and is no longer part of the encoded data.

EncodeMsg, Msg.Decode have been updated to match.
Code that decodes RLP directly from Msg.Payload will need
to change.

p2p/server_test.go

@@ -150,7 +150,7 @@ func TestServerBroadcast(t *testing.T) {
 
 	// broadcast one message
 	srv.Broadcast("discard", 0, "foo")
-	golden := unhex("66e94e166f0a2c3b884cfa59ca34")
+	golden := unhex("66e94d166f0a2c3b884cfa59ca34")
 
 	// check that the message has been written everywhere
 	for i, conn := range conns {