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.
@jonboulle I want the API to always be available, but respond differently depending on whether or not it is happy. Therefore, it needs to start serving immediately, but it should only "work" when etcd is available.