multi/refactor: separate methods for get and set node announcement

In preparation for a more complex function signature for set node
announcement, separate get and set so that readonly callers don't need
to handle the extra arguments.

lnrpc/peersrpc/config_active.go

@@ -18,7 +18,7 @@ import (
 type Config struct {
 	// GetNodeAnnouncement is used to send our retrieve the current
 	// node announcement information.
-	GetNodeAnnouncement func() (lnwire.NodeAnnouncement, error)
+	GetNodeAnnouncement func() lnwire.NodeAnnouncement
 
 	// ParseAddr parses an address from its string format to a net.Addr.
 	ParseAddr func(addr string) (net.Addr, error)

lnrpc/peersrpc/peers_server.go

@@ -313,11 +313,7 @@ func (s *Server) UpdateNodeAnnouncement(_ context.Context,
 	resp := &NodeAnnouncementUpdateResponse{}
 	nodeModifiers := make([]netann.NodeAnnModifier, 0)
 
-	currentNodeAnn, err := s.cfg.GetNodeAnnouncement()
-	if err != nil {
-		return nil, fmt.Errorf("unable to get current node "+
-			"announcement: %v", err)
-	}
+	currentNodeAnn := s.cfg.GetNodeAnnouncement()
 
 	if len(req.FeatureUpdates) > 0 {
 		features, ops, err := s.updateFeatures(