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-docs/rst/scala/code/docs/serialization/SerializationDocSpec.scala

@@ -4,7 +4,7 @@
 
 package docs.serialization {
 
-  import akka.actor.{ExtensionId, ExtensionIdProvider}
+  import akka.actor.{ ExtensionId, ExtensionIdProvider }
   import akka.testkit._
   //#imports
   import akka.actor.{ ActorRef, ActorSystem }