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/main/scala/akka/cluster/ddata/Key.scala

@@ -11,6 +11,8 @@ object Key {
 
   private[akka]type KeyR = Key[ReplicatedData]
 
+  type KeyId = String
+
 }
 
 /**
@@ -21,7 +23,7 @@ object Key {
  * Specific classes are provided for the built in data types, e.g. [[ORSetKey]],
  * and you can create your own keys.
  */
-abstract class Key[+T <: ReplicatedData](val id: String) extends Serializable {
+abstract class Key[+T <: ReplicatedData](val id: Key.KeyId) extends Serializable {
 
   override final def equals(o: Any): Boolean = o match {
     case k: Key[_] ⇒ id == k.id