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/gossiper.go

@@ -93,6 +93,10 @@ type chanPolicyUpdateRequest struct {
 	errChan       chan error
 }
 
+// PinnedSyncers is a set of node pubkeys for which we will maintain an active
+// syncer at all times.
+type PinnedSyncers map[route.Vertex]struct{}
+
 // Config defines the configuration for the service. ALL elements within the
 // configuration MUST be non-nil for the service to carry out its duties.
 type Config struct {