config+peer: replace NoChanUpdates flag with NumGraphSyncPeers

In this commit, we replace the NoChanUpdates flag with a flag that
allows us to specify the number of peers we want to actively receive new
graph updates from. This will be required when integrating the new
gossiper SyncManager subsystem with the rest of lnd.

peer.go

@@ -402,7 +402,7 @@ func (p *peer) initGossipSync() {
 		//
 		// TODO(roasbeef): craft s.t. we only get updates from a few
 		// peers
-		recvUpdates := !cfg.NoChanUpdates
+		recvUpdates := cfg.NumGraphSyncPeers != 0
 
 		// Register the this peer's for gossip syncer with the gossiper.
 		// This is blocks synchronously to ensure the gossip syncer is