server: add new shouldRequestGraphSync method to gate third feature bit

In this commit, we add a new method shouldRequestGraphSync which the
server will use in order to determine if we should request a full
channel graph sync from a newly connected remote peer. Atm, we’ll only
request a full sync iff, we have less than two peers. This is only the
initial basic logic, as we’ll later extend this to be more
comprehensive.

With this change, we’ll no longer be blasted by full channel graph
dumps for _each_ new connection after we deem that we’ve been
sufficiently bootstrapped to the network.

server.go

@@ -1079,6 +1079,17 @@ func (s *server) peerTerminationWatcher(p *peer) {
 	}
 }
 
+// shouldRequestGraphSync returns true if the servers deems it necessary that
+// we sync channel graph state with the remote peer. This method is used to
+// avoid _always_ syncing channel graph state with each peer that connects.
+//
+// NOTE: This MUST be called with the server's mutex held.
+func (s *server) shouldRequestGraphSync() bool {
+	// Initially, we'll only request a graph sync iff we have less than two
+	// peers.
+	return len(s.peersByPub) <= 2
+}
+
 // peerConnected is a function that handles initialization a newly connected
 // peer by adding it to the server's global list of all active peers, and
 // starting all the goroutines the peer needs to function properly.
@@ -1095,6 +1106,13 @@ func (s *server) peerConnected(conn net.Conn, connReq *connmgr.ConnReq,
 	// With the brontide connection established, we'll now craft the local
 	// feature vector to advertise to the remote node.
 	localFeatures := lnwire.NewRawFeatureVector()
+
+	// We'll only request a full channel graph sync if we detect that that
+	// we aren't fully synced yet.
+	if s.shouldRequestGraphSync() {
+		localFeatures.Set(lnwire.InitialRoutingSync)
+	}
+
 	// Now that we've established a connection, create a peer, and it to
 	// the set of currently active peers.
 	p, err := newPeer(conn, connReq, s, peerAddr, inbound, localFeatures)