!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
+clu #3612 Allow join to uninitialized node

* join to self not needed when performing manual joining
Merge pull request #1727 from akka/wip-3583-timer-auto-down-patriknw

!act #3583 Timer based auto-down
Merge pull request #1738 from akka/wip-3612-join-self-patriknw

+clu #3612 Allow join to uninitialized node
Merge remote-tracking branch: upstream/master