multi: rename ChannelUpdate to ChannelUpdate1

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

server.go

@@ -1753,7 +1753,7 @@ func (s *server) UpdateRoutingConfig(cfg *routing.MissionControlConfig) {
 // signAliasUpdate takes a ChannelUpdate and returns the signature. This is
 // used for option_scid_alias channels where the ChannelUpdate to be sent back
 // may differ from what is on disk.
-func (s *server) signAliasUpdate(u *lnwire.ChannelUpdate) (*ecdsa.Signature,
+func (s *server) signAliasUpdate(u *lnwire.ChannelUpdate1) (*ecdsa.Signature,
 	error) {
 
 	data, err := u.DataToSign()
@@ -4826,10 +4826,10 @@ func (s *server) fetchNodeAdvertisedAddrs(pub *btcec.PublicKey) ([]net.Addr, err
 // fetchLastChanUpdate returns a function which is able to retrieve our latest
 // channel update for a target channel.
 func (s *server) fetchLastChanUpdate() func(lnwire.ShortChannelID) (
-	*lnwire.ChannelUpdate, error) {
+	*lnwire.ChannelUpdate1, error) {
 
 	ourPubKey := s.identityECDH.PubKey().SerializeCompressed()
-	return func(cid lnwire.ShortChannelID) (*lnwire.ChannelUpdate, error) {
+	return func(cid lnwire.ShortChannelID) (*lnwire.ChannelUpdate1, error) {
 		info, edge1, edge2, err := s.graphBuilder.GetChannelByID(cid)
 		if err != nil {
 			return nil, err
@@ -4844,7 +4844,7 @@ func (s *server) fetchLastChanUpdate() func(lnwire.ShortChannelID) (
 // applyChannelUpdate applies the channel update to the different sub-systems of
 // the server. The useAlias boolean denotes whether or not to send an alias in
 // place of the real SCID.
-func (s *server) applyChannelUpdate(update *lnwire.ChannelUpdate,
+func (s *server) applyChannelUpdate(update *lnwire.ChannelUpdate1,
 	op *wire.OutPoint, useAlias bool) error {
 
 	var (