channeldb+server: prune link nodes on startup

In this commit, we extend the server's functionality to prune link nodes
on startup. Since we currently only decide whether to prune a link node
from the database based on a channel close, it's possible that we have
link nodes lingering from before this functionality was added on.

server.go

@@ -759,7 +759,12 @@ func (s *server) Start() error {
 
 	// With all the relevant sub-systems started, we'll now attempt to
 	// establish persistent connections to our direct channel collaborators
-	// within the network.
+	// within the network. Before doing so however, we'll prune our set of
+	// link nodes found within the database to ensure we don't reconnect to
+	// any nodes we no longer have open channels with.
+	if err := s.chanDB.PruneLinkNodes(); err != nil {
+		return err
+	}
 	if err := s.establishPersistentConnections(); err != nil {
 		return err
 	}