Tell unregistered workers that they are unregistered

@Shopify/data-engineers what do you guys think about doing this?

Right now an unregistered worker will continue to send heartbeats to Master which
will do nothing besides log a message that it received a heartbeat from an
unregistered worker. This change makes the Master tell unregistered workers that
they are unregistered and then the Worker will terminate. The idea being that a
unregistered worker is in a bad state anyways so we rather just termiante it, let
the watchdog restart it and then register a fresh happy worker with the master.