Stop unregistered workers.

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 which in turn makes the Worker terminate. The idea being that
an unregistered worker is in a bad state anyways so we rather just terminate it, let
some watchdog or something/someone restart it and then register a fresh happy
worker with the master.