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.
What about having an `api.Expose(chan bool)` routine which basically just does `api.available(); <- stop; api.unavailable()` ?