nsqlookupd: register/unregister cleanup

* add /debug endpoint for tests
* add UNREGISTER test
* actually remove topic registrations
* cleanup channel registrations (but warn) when unregistering a topic
* improve AddProducer/RemoveProducer so we can log more clearly when
  things are happening (and also improve general logging)
nsqd: move responsibility of notifying to topic/channel Delete()
test: expand cluster tests

* add a bit of sleep in between testing daemons that just started