+cdd #16799 Add Distributed Data module

Previously know as [patriknw/akka-data-replication](https://github.com/patriknw/akka-data-replication),
which was originally inspired by [jboner/akka-crdt](https://github.com/jboner/akka-crdt).

The functionality is very similar to akka-data-replication 0.11.

Here is a list of the most important changes:

* The package name changed to `akka.cluster.ddata`
* The extension was renamed to `DistributedData`
* The keys changed from strings to classes with unique identifiers and type information of the data values,
  e.g. `ORSetKey[Int]("set2")`
* The optional read consistency parameter was removed from the `Update` message. If you need to read from
  other replicas before performing the update you have to first send a `Get` message and then continue with
  the ``Update`` when the ``GetSuccess`` is received.
* `BigInt` is used in `GCounter` and `PNCounter` instead of `Long`
* Improvements of java api
* Better documentation

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

@@ -0,0 +1,45 @@
+/**
+ * Copyright (C) 2009-2015 Typesafe Inc. <http://www.typesafe.com>
+ */
+package akka.cluster.ddata
+
+import akka.actor.Address
+import akka.cluster.Member
+import akka.cluster.UniqueAddress
+
+/**
+ * INTERNAL API
+ */
+private[akka] object PruningState {
+  sealed trait PruningPhase
+  final case class PruningInitialized(seen: Set[Address]) extends PruningPhase
+  case object PruningPerformed extends PruningPhase
+}
+
+/**
+ * INTERNAL API
+ */
+private[akka] final case class PruningState(owner: UniqueAddress, phase: PruningState.PruningPhase) {
+  import PruningState._
+
+  def merge(that: PruningState): PruningState =
+    (this.phase, that.phase) match {
+      case (PruningPerformed, _) ⇒ this
+      case (_, PruningPerformed) ⇒ that
+      case (PruningInitialized(thisSeen), PruningInitialized(thatSeen)) ⇒
+        if (this.owner == that.owner)
+          copy(phase = PruningInitialized(thisSeen ++ thatSeen))
+        else if (Member.addressOrdering.compare(this.owner.address, that.owner.address) > 0)
+          that
+        else
+          this
+    }
+
+  def addSeen(node: Address): PruningState = phase match {
+    case PruningInitialized(seen) ⇒
+      if (seen(node) || owner.address == node) this
+      else copy(phase = PruningInitialized(seen + node))
+    case _ ⇒ this
+  }
+}
+