hack(server): 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.

The term of a denied vote is MaxUint64. The notification of the removal
is a raft event. These two modification are quick heck.

In reaction to this notification the machine should shutdown. In this
case the shutdown just moves it towards becoming a standby server.
tests(remove_node): add TestRemovePausedNode
Merge pull request #19 from unihorn/92

tests(remove_node): add TestRemovePausedNode
Merge pull request #799 from xiangli-cmu/deny_unknow_peer

hack(server): notify removed peers when they try to become candidates