introduce base64 random names

- remove Props.randomName and associated logic
- ActorRefFactory contains AtomicLong which is used to generate unique
  children names
- base64-like encoding is used with reverse “digit” order
- disallow given names which are null, empty or start with ‘$’
- random names start have ‘$’ prepended (‘$’ not being one of the 64
  characters)
- special marker “$_” for tempPath until “/tmp” supervisor is introduced
- TestActorRef uses globally unique “$$” prefix, as it creates actors
  beneath any supervisor as instructed by the user

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

@@ -65,7 +65,7 @@ class NetworkEventStream(val app: ActorSystem) {
   // FIXME: check that this supervision is correct
   private[akka] val sender = app.provider.actorOf(
     Props[Channel].copy(dispatcher = app.dispatcherFactory.newPinnedDispatcher("NetworkEventStream")),
-    app.guardian, Props.randomName, systemService = true)
+    app.systemGuardian, "network-event-sender", systemService = true)
 
   /**
    * Registers a network event stream listener (asyncronously).