Cluster: update the node configEpoch when newer is detected.
Cluster: fsync data when saving the cluster config.
Cluster: when upading the configEpoch for a node, save config on disk ASAP.
Cluster: re-order failover operations to make it safer.

We need to:

1) Increment the configEpoch.
2) Save it to disk and fsync the file.
3) Broadcast the PONG with the new configuration.

If other nodes will receive the updated configuration we need to be sure
to restart with this new config in the event of a crash.