nsqd: deleting a "live" topic

there appears to be an issue with deleting a topic (like via `nsqadmin`) when a topic that is still producing messages.

ie. the producers never return to the "active" state in `nsqlookupd` despite continuing to have the topic and produce messages.

it appears that `nsqd` is sending out of order `REGISTER`/`UNREGISTER` commands, presumably because they are just spawned goroutines and there is no guarantee which gets runtime cycles first.

cc @jehiah @elubow