= #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/Remoting.scala

@@ -231,7 +231,10 @@ private[remote] class Remoting(_system: ExtendedActorSystem, _provider: RemoteAc
   private[akka] def boundAddresses: Map[String, Set[Address]] = {
     transportMapping.map {
       case (scheme, transports) ⇒
-        scheme -> transports.map { case (transport, _) ⇒ transport.boundAddress }
+        scheme -> transports.flatMap {
+          // Need to do like this for binary compatibility reasons
+          case (t, _) ⇒ Option(t.boundAddress)
+        }
     }
   }
 }