nsqd: deleting a "live" topic

there appears to be an issue with deleting a topic (like via `nsqadmin`) when that topic is still producing messages.

ie. the producers "disappear" from the list 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