!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
Merge pull request #1727 from akka/wip-3583-timer-auto-down-patriknw

!act #3583 Timer based auto-down