!str,htc replace and remove OneBoundedInterpreter

main work by @drewhk with contributions from @2m and @rkuhn

This work uncovered many well-hidden bugs in existing Stages, in
particular StatefulStage. These were hidden by the behavior of
OneBoundedInterpreter that normally behaves more orderly than it
guarantees in general, especially with respect to the timeliness of
delivery of upstream termination signals; the bugs were then that
internal state was not flushed when onComplete arrived “too early”.

akka-docs-dev/rst/scala/code/docs/stream/cookbook/RecipeSimpleDrop.scala

@@ -2,8 +2,9 @@ package docs.stream.cookbook
 
 import akka.stream.scaladsl.{ Flow, Sink, Source }
 import akka.stream.testkit._
-
 import scala.concurrent.duration._
+import akka.testkit.TestLatch
+import scala.concurrent.Await
 
 class RecipeSimpleDrop extends RecipeSpec {
 
@@ -15,13 +16,16 @@ class RecipeSimpleDrop extends RecipeSpec {
       val droppyStream: Flow[Message, Message, Unit] =
         Flow[Message].conflate(seed = identity)((lastMessage, newMessage) => newMessage)
       //#simple-drop
+      val latch = TestLatch(2)
+      val realDroppyStream =
+        Flow[Message].conflate(seed = identity)((lastMessage, newMessage) => { latch.countDown(); newMessage })
 
       val pub = TestPublisher.probe[Message]()
       val sub = TestSubscriber.manualProbe[Message]()
       val messageSource = Source(pub)
       val sink = Sink(sub)
 
-      messageSource.via(droppyStream).to(sink).run()
+      messageSource.via(realDroppyStream).to(sink).run()
 
       val subscription = sub.expectSubscription()
       sub.expectNoMsg(100.millis)
@@ -30,6 +34,8 @@ class RecipeSimpleDrop extends RecipeSpec {
       pub.sendNext("2")
       pub.sendNext("3")
 
+      Await.ready(latch, 1.second)
+
       subscription.request(1)
       sub.expectNext("3")