notify removed peers when they try to become candidates

A peer might be removed during a network partiton. When it comes back it
will not have received any of the log entries that would have notified it
of its removal and go onto propose a vote. This will disrupt the cluster
and the cluster should give the machine feedback that it is no longer a
member.

In reaction to this notification the machine should shutdown. In this case
the shutdown just moves it towards becoming a standby server.