multi/refactor: move node ann modification outside of sign method

In preparation for moving feature bit modification inside of the
feature manager, separate node modification from signing.

netann/node_announcement.go

@@ -60,18 +60,11 @@ func NodeAnnSetTimestamp(nodeAnn *lnwire.NodeAnnouncement) {
 	nodeAnn.Timestamp = newTimestamp
 }
 
-// SignNodeAnnouncement applies the given modifies to the passed
+// SignNodeAnnouncement signs the lnwire.NodeAnnouncement provided, which
-// lnwire.NodeAnnouncement, then signs the resulting announcement. The provided
+// should be the most recent, valid update, otherwise the timestamp may not
-// update should be the most recent, valid update, otherwise the timestamp may
+// monotonically increase from the prior.
-// not monotonically increase from the prior.
 func SignNodeAnnouncement(signer lnwallet.MessageSigner,
-	keyLoc keychain.KeyLocator, nodeAnn *lnwire.NodeAnnouncement,
+	keyLoc keychain.KeyLocator, nodeAnn *lnwire.NodeAnnouncement) error {
-	mods ...NodeAnnModifier) error {
-
-	// Apply the requested changes to the node announcement.
-	for _, modifier := range mods {
-		modifier(nodeAnn)
-	}
 
 	// Create the DER-encoded ECDSA signature over the message digest.
 	sig, err := SignAnnouncement(signer, keyLoc, nodeAnn)

server.go

@@ -2961,11 +2961,15 @@ func (s *server) genNodeAnnouncement(refresh bool,
 	// propagates.
 	modifiers = append(modifiers, netann.NodeAnnSetTimestamp)
 
+	// Apply the requested changes to the node announcement.
+	for _, modifier := range modifiers {
+		modifier(s.currentNodeAnn)
+	}
+
 	// Otherwise, we'll sign a new update after applying all of the passed
 	// modifiers.
 	err := netann.SignNodeAnnouncement(
 		s.nodeSigner, s.identityKeyLoc, s.currentNodeAnn,
-		modifiers...,
 	)
 	if err != nil {
 		return lnwire.NodeAnnouncement{}, err