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/test/scala/akka/remote/RemoteDeathWatchSpec.scala

@@ -0,0 +1,33 @@
+/**
+ * Copyright (C) 2009-2011 Scalable Solutions AB <http://scalablesolutions.se>
+ */
+package akka.remote
+
+import akka.testkit._
+import akka.actor.{ ActorSystem, DeathWatchSpec }
+import com.typesafe.config.ConfigFactory
+
+@org.junit.runner.RunWith(classOf[org.scalatest.junit.JUnitRunner])
+class RemoteDeathWatchSpec extends AkkaSpec(ConfigFactory.parseString("""
+akka {
+    actor {
+        provider = "akka.remote.RemoteActorRefProvider"
+        deployment {
+            /watchers.remote = "akka://other@127.0.0.1:2666"
+        }
+    }
+    cluster.nodename = buh
+    remote.server {
+        hostname = "127.0.0.1"
+        port = 2665
+    }
+}
+""")) with ImplicitSender with DefaultTimeout with DeathWatchSpec {
+
+  val other = ActorSystem("other", ConfigFactory.parseString("akka.remote.server.port=2666").withFallback(system.settings.config))
+
+  override def atTermination() {
+    other.shutdown()
+  }
+
+}