* number-of-contacts is by default 3, and in this test
with 4 server nodes we shutdown all but one in the end
and sometimes the client has all other except the remaining
node in its list of contacts, so it will never get contact
with the remaining node
* 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
* because it will result in quarantine if failure
detection triggers and that kind of coupling is
exactly what is not desired for a ClusterClient
* replace by simple heartbeat failure detection,
DeadlineFailureDetector
* DeadLetterSuppression
* addunidoc task via an AutoPlugin that depends on PrValidation and Unidoc autoplugins
* separate cli option logic to a case class
* remove autoplugin for root project
