multi: rename ChannelUpdate to ChannelUpdate1

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

funding/manager_test.go

@@ -1210,7 +1210,7 @@ func assertChannelAnnouncements(t *testing.T, alice, bob *testNode,
 			switch m := msg.(type) {
 			case *lnwire.ChannelAnnouncement1:
 				gotChannelAnnouncement = true
-			case *lnwire.ChannelUpdate:
+			case *lnwire.ChannelUpdate1:
 
 				// The channel update sent by the node should
 				// advertise the MinHTLC value required by the