discovery: add support for PinnedSyncers

A pinned syncer is an ActiveSyncer that is configured to always remain
active for the lifetime of the connection. Pinned syncers do not count
towards the total NumActiveSyncer count, which are rotated periodically.

This features allows nodes to more tightly synchronize their routing
tables by ensuring they are always receiving gossip from distinguished
subset of peers.

discovery/syncer.go

@@ -42,6 +42,13 @@ const (
 	// They are started in a chansSynced state in order to accomplish their
 	// responsibilities above.
 	PassiveSync
+
+	// PinnedSync denotes an ActiveSync that doesn't count towards the
+	// default active syncer limits and is always active throughout the
+	// duration of the peer's connection. Each pinned syncer will begin by
+	// performing a historical sync to ensure we are well synchronized with
+	// their routing table.
+	PinnedSync
 )
 
 // String returns a human readable string describing the target SyncerType.
@@ -51,11 +58,24 @@ func (t SyncerType) String() string {
 		return "ActiveSync"
 	case PassiveSync:
 		return "PassiveSync"
+	case PinnedSync:
+		return "PinnedSync"
 	default:
 		return fmt.Sprintf("unknown sync type %d", t)
 	}
 }
 
+// IsActiveSync returns true if the SyncerType should set a GossipTimestampRange
+// allowing new gossip messages to be received from the peer.
+func (t SyncerType) IsActiveSync() bool {
+	switch t {
+	case ActiveSync, PinnedSync:
+		return true
+	default:
+		return false
+	}
+}
+
 // syncerState is an enum that represents the current state of the GossipSyncer.
 // As the syncer is a state machine, we'll gate our actions based off of the
 // current state and the next incoming message.
@@ -560,7 +580,9 @@ func (g *GossipSyncer) channelGraphSyncer() {
 			// If we haven't yet sent out our update horizon, and
 			// we want to receive real-time channel updates, we'll
 			// do so now.
-			if g.localUpdateHorizon == nil && syncType == ActiveSync {
+			if g.localUpdateHorizon == nil &&
+				syncType.IsActiveSync() {
+
 				err := g.sendGossipTimestampRange(
 					time.Now(), math.MaxUint32,
 				)
@@ -1418,7 +1440,7 @@ func (g *GossipSyncer) handleSyncTransition(req *syncTransitionReq) error {
 	switch req.newSyncType {
 	// If an active sync has been requested, then we should resume receiving
 	// new graph updates from the remote peer.
-	case ActiveSync:
+	case ActiveSync, PinnedSync:
 		firstTimestamp = time.Now()
 		timestampRange = math.MaxUint32