While we're only reacting to containers going away, it's more
convenient to bundle the docker events watcher into the server.
I've added a flag to switch it on; and switched it on by default in
the container.
I also implemented `Zone.DeleteRecordsFor(ident)`, which removes all
records for a particular container.
One minor change to the structure of the code is that the Zone is now
constructed *outside* the server, and given to it and the updater.
Michael Bridgen