=cdd #18768 Cache serialization of read and write msg

The WriteAggregator and ReadAggregator typically send
the same message to several replicas and by caching the serialized bytes
we avoid doing the same thing for each message

and add a test for WriteAggregator

akka-distributed-data/src/main/scala/akka/cluster/ddata/Replicator.scala

@@ -1359,7 +1359,7 @@ private[akka] class WriteAggregator(
 
   def receive = {
     case WriteAck ⇒
-      remaining -= sender().path.address
+      remaining -= senderAddress()
       if (remaining.size == doneWhenRemainingSize)
         reply(ok = true)
     case SendToSecondary ⇒
@@ -1367,6 +1367,8 @@ private[akka] class WriteAggregator(
     case ReceiveTimeout ⇒ reply(ok = false)
   }
 
+  def senderAddress(): Address = sender().path.address
+
   def reply(ok: Boolean): Unit = {
     if (ok && envelope.data == DeletedData)
       replyTo.tell(DeleteSuccess(key), context.parent)