Remote DeathWatch (2): make it work

- introduce EmptyLocalActorRef, which is returned for unsuccessful
  look-ups of local scope
- this fixes the problem that actors—after their death—can still be
  looked up without losing their identity; otherwise behave like
  DeadLetterActorRef
- adapt tests accordingly
- make DeathWatchSpec reusable and build remote test from it
- remove several unused imports of LocalActorRef
- use LocalRef/RemoteRef in pattern matches where applicable: these are
  marker traits for a ref’s scope; InternalActorRef mandates a scope as
  per its self-type

akka-remote/src/main/scala/akka/remote/RemoteActorRefProvider.scala

@@ -31,7 +31,6 @@ class RemoteActorRefProvider(
 
   val remoteSettings = new RemoteSettings(settings.config, systemName)
 
-  def deathWatch = local.deathWatch
   def rootGuardian = local.rootGuardian
   def guardian = local.guardian
   def systemGuardian = local.systemGuardian
@@ -49,6 +48,8 @@ class RemoteActorRefProvider(
 
   private val local = new LocalActorRefProvider(systemName, settings, eventStream, scheduler, _deadLetters, rootPath, deployer)
 
+  val deathWatch = new RemoteDeathWatch(local.deathWatch, this)
+
   def init(system: ActorSystemImpl) {
     local.init(system)
     remote.init(system, this)
@@ -150,6 +151,10 @@ class RemoteActorRefProvider(
   }
 }
 
+trait RemoteRef extends ActorRefScope {
+  final def isLocal = false
+}
+
 /**
  * Remote ActorRef that is used when referencing the Actor on a different node than its "home" node.
  * This reference is network-aware (remembers its origin) and immutable.
@@ -160,7 +165,7 @@ private[akka] class RemoteActorRef private[akka] (
   val path: ActorPath,
   val getParent: InternalActorRef,
   loader: Option[ClassLoader])
-  extends InternalActorRef {
+  extends InternalActorRef with RemoteRef {
 
   def getChild(name: Iterator[String]): InternalActorRef = {
     val s = name.toStream
@@ -206,11 +211,11 @@ private[akka] class RemoteActorRef private[akka] (
 class RemoteDeathWatch(val local: LocalDeathWatch, val provider: RemoteActorRefProvider) extends DeathWatch {
 
   def subscribe(watcher: ActorRef, watched: ActorRef): Boolean = watched match {
-    case r: RemoteActorRef ⇒
+    case r: RemoteRef ⇒
       val ret = local.subscribe(watcher, watched)
       provider.actorFor(r.path.root / "remote") ! DaemonMsgWatch(watcher, watched)
       ret
-    case l: LocalActorRef ⇒
+    case l: LocalRef ⇒
       local.subscribe(watcher, watched)
     case _ ⇒
       provider.log.error("unknown ActorRef type {} as DeathWatch target", watched.getClass)