Current implementation of deleteTopic has few usability issues:
1. Successful deleteTopic doesn't give the user any feedback that anything happened
2. Attempting to delete a topic twice leads to a "node exists" exception from zkclient. We should catch this and send "Topic is being deleted" error to the user.
3. Listing topics may show deleted topics (while the brokers are still cleaning up), it will be nice to mark them as deleted.