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-remote/src/main/scala/akka/remote/netty/Settings.scala

@@ -38,7 +38,15 @@ class NettySettings(config: Config, val systemName: String) {
     case ""    ⇒ InetAddress.getLocalHost.getHostAddress
     case value ⇒ value
   }
-  val Port = getInt("port")
+  val Port = getInt("port") match {
+    case 0 ⇒
+      try {
+        val s = new java.net.ServerSocket(0)
+        try s.getLocalPort finally s.close()
+      } catch { case e ⇒ throw new ConfigurationException("Unable to obtain random port", e) }
+    case other ⇒ other
+  }
+
   val ConnectionTimeout = Duration(getMilliseconds("connection-timeout"), MILLISECONDS)
 
   val Backlog = getInt("backlog")