rework system initialization, remove remote address from LocalActorRef

- move all creation of ActorRefs into the ActorRefProvider (deadLetters,
  locker)
- rootPath does not contain remote transport address any longer in order
  to start the LocalActorRefProvider before the RemoteTransport; the
  transport address is inserted during serialization only, which enables
  us later to have more than one transport available for one actor
  system (maybe even needed for clustering)
- fix inheritance between DeadLetterActorRef and EmptyLocalActorRef
- document some start-up dependencies by reordering constructor code
- fix remote tests which used self.path for identifying remote actors
  (since that no longer includes the remote transport address)

akka-actor/src/main/scala/akka/serialization/Serialization.scala

@@ -9,7 +9,7 @@ import akka.util.ReflectiveAccess
 import scala.util.DynamicVariable
 import com.typesafe.config.Config
 import akka.config.ConfigurationException
-import akka.actor.{ Extension, ActorSystem, ExtendedActorSystem }
+import akka.actor.{ Extension, ActorSystem, ExtendedActorSystem, Address }
 
 case class NoSerializerFoundException(m: String) extends AkkaException(m)
 
@@ -27,6 +27,12 @@ object Serialization {
    */
   val currentSystem = new DynamicVariable[ActorSystem](null)
 
+  /**
+   * This holds a reference to the current transport address to be inserted
+   * into local actor refs during serialization.
+   */
+  val currentTransportAddress = new DynamicVariable[Address](null)
+
   class Settings(val config: Config) {
 
     import scala.collection.JavaConverters._