A subscribe request must specify zero or one topic. So suppose several
clients are subscribed to the same topic.
1) If all clients unsubscribe, does Muse still hold on to the topic? If
so, how do we prevent Muse from building up a list of unused topics, if
it does keep such a list?
2) Is there a way to destroy/pause a topic, and effectively stop its
corresponding notifications from being sent out, without killing the
subscriptions? Then be able to resume the topic and notifications at a
later time?
