improve peer gc efficiency

Instead of separately calculating routes to every individual peer,
carry out one calculation for routes to all peers. Same as unicast
routes except taking into account asymmetric connections.

router/peers.go

@@ -147,9 +147,9 @@ func (peers *Peers) fetchAlias(peer *Peer) (*Peer, bool) {
 
 func (peers *Peers) garbageCollect() []*Peer {
 	removed := []*Peer{}
+	_, reachable := peers.ourself.Routes(nil, false)
 	for name, peer := range peers.table {
-		found, _ := peers.ourself.Routes(peer, false)
-		if !found && !peer.IsLocallyReferenced() {
+		if _, found := reachable[peer.Name]; !found && !peer.IsLocallyReferenced() {
 			delete(peers.table, name)
 			peers.onGC(peer)
 			removed = append(removed, peer)