+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-docs/rst/scala/code/docs/ddata/protobuf/TwoPhaseSetSerializer2.scala

@@ -0,0 +1,59 @@
+/**
+ * Copyright (C) 2015 Typesafe Inc. <http://www.typesafe.com>
+ */
+package docs.ddata.protobuf
+
+//#serializer
+import scala.collection.JavaConverters._
+import akka.actor.ExtendedActorSystem
+import akka.cluster.ddata.GSet
+import akka.cluster.ddata.protobuf.ReplicatedDataSerializer
+import akka.cluster.ddata.protobuf.SerializationSupport
+import akka.serialization.Serializer
+import docs.ddata.TwoPhaseSet
+import docs.ddata.protobuf.msg.TwoPhaseSetMessages
+
+class TwoPhaseSetSerializer2(val system: ExtendedActorSystem)
+  extends Serializer with SerializationSupport {
+
+  override def includeManifest: Boolean = false
+
+  override def identifier = 99999
+
+  val replicatedDataSerializer = new ReplicatedDataSerializer(system)
+
+  override def toBinary(obj: AnyRef): Array[Byte] = obj match {
+    case m: TwoPhaseSet ⇒ twoPhaseSetToProto(m).toByteArray
+    case _ ⇒ throw new IllegalArgumentException(
+      s"Can't serialize object of type ${obj.getClass}")
+  }
+
+  override def fromBinary(bytes: Array[Byte], clazz: Option[Class[_]]): AnyRef = {
+    twoPhaseSetFromBinary(bytes)
+  }
+
+  def twoPhaseSetToProto(twoPhaseSet: TwoPhaseSet): TwoPhaseSetMessages.TwoPhaseSet2 = {
+    val b = TwoPhaseSetMessages.TwoPhaseSet2.newBuilder()
+    if (!twoPhaseSet.adds.isEmpty)
+      b.setAdds(otherMessageToProto(twoPhaseSet.adds).toByteString())
+    if (!twoPhaseSet.removals.isEmpty)
+      b.setRemovals(otherMessageToProto(twoPhaseSet.removals).toByteString())
+    b.build()
+  }
+
+  def twoPhaseSetFromBinary(bytes: Array[Byte]): TwoPhaseSet = {
+    val msg = TwoPhaseSetMessages.TwoPhaseSet2.parseFrom(bytes)
+    val adds =
+      if (msg.hasAdds)
+        otherMessageFromBinary(msg.getAdds.toByteArray).asInstanceOf[GSet[String]]
+      else
+        GSet.empty[String]
+    val removals =
+      if (msg.hasRemovals)
+        otherMessageFromBinary(msg.getRemovals.toByteArray).asInstanceOf[GSet[String]]
+      else
+        GSet.empty[String]
+    TwoPhaseSet(adds, removals)
+  }
+}
+//#serializer