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.
any particular reason for the signature to not return a `Heart`? (I realise you're casting it in `newHeartMonitorFromConfig` but wonder why not here)