add Join/Leave to PubSubRouter interface

advanced routers will want to know when they are subscribed to a topic as
they may want to send control messages.

floodsub.go

@@ -78,3 +78,7 @@ func (fs *FloodSubRouter) Publish(from peer.ID, msg *pb.Message) {
 		}
 	}
 }
+
+func (fs *FloodSubRouter) Join(topic string) {}
+
+func (fs *FloodSubRouter) Leave(topic string) {}

pubsub.go

@@ -96,6 +96,8 @@ type PubSubRouter interface {
 	RemovePeer(peer.ID)
 	HandleRPC(*RPC)
 	Publish(peer.ID, *pb.Message)
+	Join(topic string)
+	Leave(topic string)
 }
 
 type Message struct {
@@ -276,6 +278,7 @@ func (p *PubSub) handleRemoveSubscription(sub *Subscription) {
 	if len(subs) == 0 {
 		delete(p.myTopics, sub.topic)
 		p.announce(sub.topic, false)
+		p.rt.Leave(sub.topic)
 	}
 }
 
@@ -290,6 +293,7 @@ func (p *PubSub) handleAddSubscription(req *addSubReq) {
 	// announce we want this topic
 	if len(subs) == 0 {
 		p.announce(sub.topic, true)
+		p.rt.Join(sub.topic)
 	}
 
 	// make new if not there