Monitor etcd connectivity

Since the heartbeat provides presence information, a failed heartbeat means the fleet server is no longer able to operate in the cluster. The Heart and Monitor pieces are now responsible for the Machine heartbeat. The Monitor provides a trigger for stopping, purging, and starting the fleet server in response to a failed heartbeat.
Never mind, didn't quite read your comment.
It might be nicer to wire up the API to leverage the `stop` chan like everything else.