multi: rename ChannelUpdate to ChannelUpdate1

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

netann/sign.go

@@ -22,7 +22,7 @@ func SignAnnouncement(signer lnwallet.MessageSigner, keyLoc keychain.KeyLocator,
 	switch m := msg.(type) {
 	case *lnwire.ChannelAnnouncement1:
 		data, err = m.DataToSign()
-	case *lnwire.ChannelUpdate:
+	case *lnwire.ChannelUpdate1:
 		data, err = m.DataToSign()
 	case *lnwire.NodeAnnouncement:
 		data, err = m.DataToSign()