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/ORMap.scala

@@ -177,6 +177,13 @@ final class ORMap[A, B <: ReplicatedData] private[akka] (
     new ORMap(mergedKeys, mergedValues)
   }
 
+  override def modifiedByNodes: Set[UniqueAddress] = {
+    keys.modifiedByNodes union values.foldLeft(Set.empty[UniqueAddress]) {
+      case (acc, (_, data: RemovedNodePruning)) ⇒ acc union data.modifiedByNodes
+      case (acc, _)                             ⇒ acc
+    }
+  }
+
   override def needPruningFrom(removedNode: UniqueAddress): Boolean = {
     keys.needPruningFrom(removedNode) || values.exists {
       case (_, data: RemovedNodePruning) ⇒ data.needPruningFrom(removedNode)