multi: rename AnnounceSignatures to AnnounceSignatures1

In preparation for adding a new message, AnnounceSignatures2 along with
an AnnounceSignatures interface, we rename the existing message to
AnnounceSignatures1.

discovery/message_store.go

@@ -83,7 +83,7 @@ func NewMessageStore(db kvdb.Backend) (*MessageStore, error) {
 func msgShortChanID(msg lnwire.Message) (lnwire.ShortChannelID, error) {
 	var shortChanID lnwire.ShortChannelID
 	switch msg := msg.(type) {
-	case *lnwire.AnnounceSignatures:
+	case *lnwire.AnnounceSignatures1:
 		shortChanID = msg.ShortChannelID
 	case *lnwire.ChannelUpdate:
 		shortChanID = msg.ShortChannelID