= #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-remote/src/main/scala/akka/remote/serialization/ProtobufSerializer.scala

@@ -39,6 +39,17 @@ object ProtobufSerializer {
  */
 class ProtobufSerializer(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) 2
+    else identifierFromConfig
+
+  @deprecated("Will be removed without replacement", "2.4")
+  val ARRAY_OF_BYTE_ARRAY = Array[Class[_]](classOf[Array[Byte]])
+
   private val parsingMethodBindingRef = new AtomicReference[Map[Class[_], Method]](Map.empty)
 
   override def includeManifest: Boolean = true
@@ -52,8 +63,9 @@ class ProtobufSerializer(val system: ExtendedActorSystem) extends BaseSerializer
           parsingMethodBinding.get(clazz) match {
             case Some(cachedParsingMethod) ⇒ cachedParsingMethod
             case None ⇒
-              import ProtobufSerializer.ARRAY_OF_BYTE_ARRAY
-              val unCachedParsingMethod = if (method eq null) clazz.getDeclaredMethod("parseFrom", ARRAY_OF_BYTE_ARRAY: _*) else method
+              val unCachedParsingMethod =
+                if (method eq null) clazz.getDeclaredMethod("parseFrom", ProtobufSerializer.ARRAY_OF_BYTE_ARRAY: _*)
+                else method
               if (parsingMethodBindingRef.compareAndSet(parsingMethodBinding, parsingMethodBinding.updated(clazz, unCachedParsingMethod)))
                 unCachedParsingMethod
               else