make pruning of CRDT garbage work, #21647

* fix merge issues of DataEnvelope and its pruning
* simplify by removing the tombstones, which didn't work in all cases anyway
* keep the PruningPerformed markers in the DataEnvelope until configured
  TTL has elapsed (wall clock)
* simplify PruningState structure
* also store the pruning markers in durable data
* collect removed nodes from the data, listing on MemberRemoved is not enough
* possibility to disable pruning altogether
* documented caveat for durable data

akka-distributed-data/src/main/scala/akka/cluster/ddata/VersionVector.scala

@@ -292,6 +292,9 @@ final case class OneVersionVector private[akka] (node: UniqueAddress, version: L
     }
   }
 
+  override def modifiedByNodes: Set[UniqueAddress] =
+    Set(node)
+
   override def needPruningFrom(removedNode: UniqueAddress): Boolean =
     node == removedNode
 
@@ -353,6 +356,9 @@ final case class ManyVersionVector(versions: TreeMap[UniqueAddress, Long]) exten
     }
   }
 
+  override def modifiedByNodes: Set[UniqueAddress] =
+    versions.keySet
+
   override def needPruningFrom(removedNode: UniqueAddress): Boolean =
     versions.contains(removedNode)