* the reported issue is fixed by the immediate leaderActions
(moving to Up) when joining the first node to itself
* the other changes are precautions just in case
* In 2.4 we derive the number of hand-over/take-over retries from
the removal margin, but we decided to set that to 0 by default, since
it is intended for network partition scenarios. maxTakeOverRetries
became 1. So there must be also be a min number of retries property.
* The test failed for the leaving scenario because the singleton
instance was stopped hard without sending the terminationMessage when
the maxTakeOverRetries was exceeded.
For manual downing it is not needed. For auto-down it doesn't add any extra safety, since that
is not handling network partitions anyway.
The setting is still useful if you implement downing strategies that handle network partitions,
e.g. by keeping the larger side of the partition and shutting down the smaller side.
* 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
