!con #3597 Remove hand over data message in cluster singleton
=clu #3608 Simplify Reachability merge

* No need to merge the individual rows for one observer, simply pick
  the rows from the observer with highest version
=clu #3533 Harden cluster startup error handling
Merge pull request #1719 from akka/wip-3597-cluster-singleton-hand-over-patriknw

!con #3597 Remove hand over data message in cluster singleton
!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 #1740 from akka/wip-3608-simplify-merge-reachability-patriknw

=clu #3608 Simplify Reachability merge
Merge pull request #1741 from akka/wip-3533-cluster-startup-failure-patriknw

=clu #3533 Harden cluster startup error handling
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
!per #3631 Snapshotting

- capture and save snapshots of processor state
- start processor recovery from saved snapshots
- snapshot storage on local filesystem
- snapshot store completely isolated from journal
- LevelDB journal modularized (and completely re-rwritten)
- In-memory journal removed
Merge pull request #1747 from eligosource/wip-3631-snapshotting-krasserm

!per #3631 Snapshotting