causal delivery of deltas, #22188

* keep track of delta interval versions and skip deltas
  that are not consequtive, i.e. when some delta message was lost
* send the delta versions in the full state gossip to sync up the
  expected versions after dropped deltas
* implementation of deltas for ORSet
* refactoring of the delta types to allow for different type for the
  delta and the full state
* extensive tests
* mima filter
* performance optimizations
* simple pruning of deltas
* Java API
* update documentation
* KeyId type alias
* Use InternalApi annotation

akka-distributed-data/src/test/java/akka/cluster/ddata/JavaImplOfDeltaReplicatedData2.java

@@ -0,0 +1,49 @@
+/**
+ * Copyright (C) 2017 Lightbend Inc. <http://www.lightbend.com>
+ */
+package akka.cluster.ddata;
+
+
+import java.util.Optional;
+
+import akka.cluster.UniqueAddress;
+
+// different delta type
+public class JavaImplOfDeltaReplicatedData2
+  extends AbstractDeltaReplicatedData<JavaImplOfDeltaReplicatedData2, JavaImplOfDeltaReplicatedData2.Delta> {
+
+  public static class Delta extends AbstractReplicatedData<Delta> implements ReplicatedDelta, RequiresCausalDeliveryOfDeltas {
+    @Override
+    public Delta mergeData(Delta other) {
+      return this;
+    }
+
+    @Override
+    public JavaImplOfDeltaReplicatedData2 zero() {
+      return new JavaImplOfDeltaReplicatedData2();
+    }
+  }
+
+  @Override
+  public JavaImplOfDeltaReplicatedData2 mergeData(JavaImplOfDeltaReplicatedData2 other) {
+    return this;
+  }
+
+  @Override
+  public JavaImplOfDeltaReplicatedData2 mergeDeltaData(Delta other) {
+    return this;
+  }
+
+  @Override
+  public Optional<Delta> deltaData() {
+    return Optional.empty();
+  }
+
+  @Override
+  public JavaImplOfDeltaReplicatedData2 resetDelta() {
+    return this;
+  }
+
+
+
+}