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-actor/src/main/scala/akka/actor/Props.scala

@@ -30,7 +30,6 @@ object Props {
   final val defaultFaultHandler: FaultHandlingStrategy = OneForOneStrategy(defaultDecider, None, None)
 
   final val noHotSwap: Stack[Actor.Receive] = Stack.empty
-  final val randomName: String = ""
 
   /**
    * The default Props instance, uses the settings from the Props object starting with default*