= #17342 Make 2.4 binary compatible with 2.3

(cherry picked from commit 89af8bdb90)

* remove final identifier in serializers

i* revert/deprecate ProtobufSerializer.ARRAY_OF_BYTE_ARRAY

* adding back compatible empty constructor in serializers

* make FSM.State compatible

* add back ActorPath.ElementRegex

* revert SocketOption changes and add SocketOptionV2
  see a6d3704ef6

* problem filter for ActorSystem and ActorPath

* problem filter for ByteString

* problem filter for deprecated Timeout methods

* BalancingPool companion

* ask

* problem filter for ActorDSL

* event bus

* exclude hasSubscriptions

* exclude some problems in testkit

* boundAddress and addressFromSocketAddress

* Pool nrOfInstances

* PromiseActorRef

* check with 2.3.9

* migration guide note

* explicit exclude of final class problems

akka-cluster/src/main/scala/akka/cluster/protobuf/ClusterMessageSerializer.scala

@@ -24,6 +24,14 @@ import scala.concurrent.duration.Deadline
  */
 class ClusterMessageSerializer(val system: ExtendedActorSystem) extends BaseSerializer {
 
+  @deprecated("Use constructor with ExtendedActorSystem", "2.4")
+  def this() = this(null)
+
+  // TODO remove this when deprecated this() is removed
+  override val identifier: Int =
+    if (system eq null) 5
+    else identifierFromConfig
+
   private final val BufferSize = 1024 * 4
   // must be lazy because serializer is initialized from Cluster extension constructor
   private lazy val GossipTimeToLive = Cluster(system).settings.GossipTimeToLive