multi: rename ChannelUpdate to ChannelUpdate1

In preparation for adding a new ChannelUpdate2 message and a
ChannelUpdate interface, we rename the existing message to
ChannelUpdate1.

lnwire/message.go

@@ -261,7 +261,7 @@ func makeEmptyMessage(msgType MessageType) (Message, error) {
 	case MsgChannelAnnouncement:
 		msg = &ChannelAnnouncement1{}
 	case MsgChannelUpdate:
-		msg = &ChannelUpdate{}
+		msg = &ChannelUpdate1{}
 	case MsgNodeAnnouncement:
 		msg = &NodeAnnouncement{}
 	case MsgPing: