* The scenario was (probably) that a node was restarted with
same host:port and then didn't join the same cluster. The DData
Replicator in the original cluster would continue sending messages
to the new incarnation resulting in false removals.
* The fix is that DData Replicator includes the system uid of the sending
or target system in messages and if recipient gets a message that is from/to
unknown it will discard it and thereby not spreading information across
different clusters.
* Reproduced in ClusterReceptionistSpec
* Much hardening of other things in ClusterReceptionistSpec
* There are also some improvements to ClusterReceptionist to not leak
Listing with refs of removed nodes.
* use ClusterShuttingDown
* The reason for using sender system uid instead of target uid in messages
like Read and Write is that then the optimization for sending same message
to many destinations can remain.
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