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/interfaces.go

@@ -86,7 +86,7 @@ type scidAliasHandler interface {
 	// HTLCs on option_scid_alias channels.
 	attachFailAliasUpdate(failClosure func(
 		sid lnwire.ShortChannelID,
-		incoming bool) *lnwire.ChannelUpdate)
+		incoming bool) *lnwire.ChannelUpdate1)
 
 	// getAliases fetches the link's underlying aliases. This is used by
 	// the Switch to determine whether to forward an HTLC and where to