=str fix sub-upstream cancellation in concatAll

- ActorProcessor terminated eagerly when ConcatAll had just taken up a
  new input stream but not yet received onSubscribe for it

- The ActorProcessor eagerly shuts itself down upon onError and that
  cannot be changed without completely reworking the Pump, so I opted
  for just tracking the outstanding substreamSubscribers that have not
  yet seen OnSubscribe and making them cancel properly when that arrives
  (possibly later).

akka-stream-tests/src/test/scala/akka/stream/impl/ActorInterpreterSpec.scala

@@ -22,7 +22,7 @@ class ActorInterpreterSpec extends AkkaSpec {
   implicit val mat = ActorFlowMaterializer()
 
   class Setup(ops: List[Stage[_, _]] = List(fusing.Map({ x: Any ⇒ x }, stoppingDecider))) {
-    val up = TestPublisher.manualProbe[Int]
+    val up = TestPublisher.manualProbe[Int]()
     val down = TestSubscriber.manualProbe[Int]
     private val props = ActorInterpreter.props(mat.settings, ops, mat).withDispatcher("akka.test.stream-dispatcher")
     val actor = system.actorOf(props)