multi: rename ChannelUpdate to ChannelUpdate1

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

htlcswitch/test_utils.go

@@ -91,8 +91,10 @@ func genIDs() (lnwire.ChannelID, lnwire.ChannelID, lnwire.ShortChannelID,
 
 // mockGetChanUpdateMessage helper function which returns topology update of
 // the channel
-func mockGetChanUpdateMessage(cid lnwire.ShortChannelID) (*lnwire.ChannelUpdate, error) {
-	return &lnwire.ChannelUpdate{
+func mockGetChanUpdateMessage(_ lnwire.ShortChannelID) (*lnwire.ChannelUpdate1,
+	error) {
+
+	return &lnwire.ChannelUpdate1{
 		Signature: wireSig,
 	}, nil
 }