!act #3583 Timer based auto-down

* 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

akka-cluster/src/multi-jvm/scala/akka/cluster/MultiNodeClusterSpec.scala

@@ -34,7 +34,6 @@ object MultiNodeClusterSpec {
   def clusterConfig: Config = ConfigFactory.parseString("""
     akka.actor.provider = akka.cluster.ClusterActorRefProvider
     akka.cluster {
-      auto-down                           = off
       jmx.enabled                         = off
       gossip-interval                     = 200 ms
       leader-actions-interval             = 200 ms