Oops.

This raises another question; currently there's no way to kill the zkmonitor. I'm not sure how the producer is used in practice, but if we're making and closing a lot of them there could be a leak here.

There's also a code duplication issue here, as zkmonitor is pretty much the same thing as the cache_children thing in gevent-server - although they are quite different in implementation. We can't reasonably make gevent-kafka depend on gevent-server, though.