* Based on Jonas' notes: https://gist.github.com/jboner/7692270
* Also removed "Future Extension" descriptions of cluster actor reference,
since that is probably not the direction we are taking
* Replace (deprecate) akka.cluster.auto-down config setting with
akka.cluster.auto-down-unreachable-after
* AutoDown actor that keeps track of unreachable members
and performs down from the leader node when they have been
unreachable for the specified duration
* Migration guide
* Replace unreachable Set with Reachability table
* Unreachable members stay in member Set
* Downing a live member was moved it to the unreachable Set,
and then removed from there by the leader. That will not
work when flipping back to reachable, so a Down member must
be detected as unreachable before beeing removed. Similar
to Exiting. Member shuts down itself if it sees itself as
Down.
* Flip back to reachable when failure detector monitors it as
available again
* ReachableMember event
* Can't ignore gossip from aggregated unreachable (see SurviveNetworkInstabilitySpec)
* Make use of ReachableMember event in cluster router
* End heartbeat when acknowledged, EndHeartbeatAck
* Remove nr-of-end-heartbeats from conf
* Full reachability info in JMX cluster status
* Don't use interval after unreachable for AccrualFailureDetector history
* Add QuarantinedEvent to remoting, used for Reachability.Terminated
* Prune reachability table when all reachable
* Update documentation
* Performance testing and optimizations
* The cluster spec has been rearranged into two sections describing the current cluster and future enhancements/additions.
* The state diagram has been replaced with a new one describing the current cluster implementation.
* A number of small additons and changes to the text.
