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.

config.go

@@ -248,7 +248,7 @@ type config struct {
 	Color       string `long:"color" description:"The color of the node in hex format (i.e. '#3399FF'). Used to customize node appearance in intelligence services"`
 	MinChanSize int64  `long:"minchansize" description:"The smallest channel size (in satoshis) that we should accept. Incoming channels smaller than this will be rejected"`
 
-	NoChanUpdates bool `long:"nochanupdates" description:"If specified, lnd will not request real-time channel updates from connected peers. This option should be used by routing nodes to save bandwidth."`
+	NumGraphSyncPeers int `long:"numgraphsyncpeers" description:"The number of peers that we should receive new graph updates from. This option can be tuned to save bandwidth for light clients or routing nodes."`
 
 	RejectPush bool `long:"rejectpush" description:"If true, lnd will not accept channel opening requests with non-zero push amounts. This should prevent accidental pushes to merchant nodes."`
 
@@ -335,6 +335,7 @@ func loadConfig() (*config, error) {
 		Alias:                    defaultAlias,
 		Color:                    defaultColor,
 		MinChanSize:              int64(minChanFundingSize),
+		NumGraphSyncPeers:        defaultMinPeers,
 		Tor: &torConfig{
 			SOCKS:   defaultTorSOCKS,
 			DNS:     defaultTorDNS,

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