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/RemoteActorRefProvider.scala

@@ -243,6 +243,8 @@ class RemoteActorRefProvider(val app: ActorSystem) extends ActorRefProvider {
   private[akka] def ask(message: Any, recipient: ActorRef, within: Timeout): Future[Any] = local.ask(message, recipient, within)
 
   private[akka] def dummyAskSender = local.dummyAskSender
+
+  private[akka] def tempPath = local.tempPath
 }
 
 /**