+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/DistributedData.scala

@@ -0,0 +1,52 @@
+/**
+ * Copyright (C) 2009-2015 Typesafe Inc. <http://www.typesafe.com>
+ */
+package akka.cluster.ddata
+
+import scala.concurrent.duration._
+
+import akka.actor.ActorRef
+import akka.actor.ActorSystem
+import akka.actor.ExtendedActorSystem
+import akka.actor.Extension
+import akka.actor.ExtensionId
+import akka.actor.ExtensionIdProvider
+import akka.cluster.Cluster
+
+object DistributedData extends ExtensionId[DistributedData] with ExtensionIdProvider {
+  override def get(system: ActorSystem): DistributedData = super.get(system)
+
+  override def lookup = DistributedData
+
+  override def createExtension(system: ExtendedActorSystem): DistributedData =
+    new DistributedData(system)
+}
+
+/**
+ * Akka extension for convenient configuration and use of the
+ * [[Replicator]]. Configuration settings are defined in the
+ * `akka.cluster.ddata` section, see `reference.conf`.
+ */
+class DistributedData(system: ExtendedActorSystem) extends Extension {
+
+  private val config = system.settings.config.getConfig("akka.cluster.distributed-data")
+  private val settings = ReplicatorSettings(config)
+
+  /**
+   * Returns true if this member is not tagged with the role configured for the
+   * replicas.
+   */
+  def isTerminated: Boolean = Cluster(system).isTerminated || !settings.role.forall(Cluster(system).selfRoles.contains)
+
+  /**
+   * `ActorRef` of the [[Replicator]] .
+   */
+  val replicator: ActorRef =
+    if (isTerminated) {
+      system.log.warning("Replicator points to dead letters: Make sure the cluster node is not terminated and has the proper role!")
+      system.deadLetters
+    } else {
+      val name = config.getString("name")
+      system.systemActorOf(Replicator.props(settings), name)
+    }
+}