multi: rename ChannelAnnouncement to ChannelAnnouncment1

In preparation for adding the new ChannelAnnouncement2 message along
with a ChannelAnnouncement interface, we rename the existing message to
ChannelAnnouncement1.

netann/sign.go

@@ -20,7 +20,7 @@ func SignAnnouncement(signer lnwallet.MessageSigner, keyLoc keychain.KeyLocator,
 	)
 
 	switch m := msg.(type) {
-	case *lnwire.ChannelAnnouncement:
+	case *lnwire.ChannelAnnouncement1:
 		data, err = m.DataToSign()
 	case *lnwire.ChannelUpdate:
 		data, err = m.DataToSign()