!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-stream/src/main/scala/akka/stream/impl/StreamLayout.scala

@@ -184,7 +184,7 @@ private[akka] object StreamLayout {
         downstreams,
         upstreams,
         Transform(f, if (this.isSealed) Atomic(this) else this.materializedValueComputation),
-        attributes)
+        if (this.isSealed) Attributes.none else attributes)
     }
 
     /**
@@ -225,7 +225,7 @@ private[akka] object StreamLayout {
         upstreams ++ that.upstreams,
         // would like to optimize away this allocation for Keep.{left,right} but that breaks side-effecting transformations
         Combine(f.asInstanceOf[(Any, Any) ⇒ Any], matComputation1, matComputation2),
-        attributes)
+        Attributes.none)
     }
 
     /**
@@ -595,7 +595,7 @@ private[stream] object MaterializerSession {
 /**
  * INTERNAL API
  */
-private[stream] abstract class MaterializerSession(val topLevel: StreamLayout.Module) {
+private[stream] abstract class MaterializerSession(val topLevel: StreamLayout.Module, val initialAttributes: Attributes) {
   import StreamLayout._
 
   private var subscribersStack: List[mutable.Map[InPort, Subscriber[Any]]] =
@@ -653,7 +653,7 @@ private[stream] abstract class MaterializerSession(val topLevel: StreamLayout.Mo
     require(
       topLevel.isRunnable,
       s"The top level module cannot be materialized because it has unconnected ports: ${(topLevel.inPorts ++ topLevel.outPorts).mkString(", ")}")
-    try materializeModule(topLevel, topLevel.attributes)
+    try materializeModule(topLevel, initialAttributes and topLevel.attributes)
     catch {
       case NonFatal(cause) ⇒
         // PANIC!!! THE END OF THE MATERIALIZATION IS NEAR!