* avoid using Down and Exiting member from being used for joining
* delay shut down of Down member until the information is spread
to all reachable members, e.g. downing several nodes via one node
* akka.cluster.down-removal-margin setting
Margin until shards or singletons that belonged to a
downed/removed partition are created in surviving partition.
Used by singleton and sharding.
* remove the retry count parameters/settings for singleton in
favor of deriving those from the removal-margin
