=str #16751 Update to reactive-streams 1.0-RC3

Changed rules:
* 1.9 Always onSubscribe prior to any other signals
* 1.9 NullPointerException if subscriber is null
* 3.17 Long overflow, effectively unbounded instead of onError

Fixed some more things:
* fixed some FIXME
* Long drop and take
* memory leaks in tck tests, use BeforeClass to create ActorSystem
  use AfterClass to shutdown ActorSystem
* more tck tests
* don't emit OnComplete when substream is cancelled
* work around for memory leak in PrefixAndTail

akka-stream-tests/src/test/scala/akka/stream/scaladsl/FlowMapAsyncSpec.scala

@@ -16,6 +16,7 @@ import akka.testkit.TestLatch
 import akka.testkit.TestProbe
 import akka.stream.scaladsl.OperationAttributes.supervisionStrategy
 import akka.stream.Supervision.resumingDecider
+import akka.stream.impl.ReactiveStreamsCompliance
 
 class FlowMapAsyncSpec extends AkkaSpec {
 
@@ -137,6 +138,25 @@ class FlowMapAsyncSpec extends AkkaSpec {
       c.expectComplete()
     }
 
+    "signal NPE when future is completed with null" in {
+      val c = StreamTestKit.SubscriberProbe[String]()
+      val p = Source(List("a", "b")).mapAsync(elem ⇒ Future.successful(null)).to(Sink(c)).run()
+      val sub = c.expectSubscription()
+      sub.request(10)
+      c.expectError.getMessage should be(ReactiveStreamsCompliance.ElementMustNotBeNullMsg)
+    }
+
+    "resume when future is completed with null" in {
+      val c = StreamTestKit.SubscriberProbe[String]()
+      val p = Source(List("a", "b", "c")).section(supervisionStrategy(resumingDecider))(
+        _.mapAsync(elem ⇒ if (elem == "b") Future.successful(null) else Future.successful(elem)))
+        .to(Sink(c)).run()
+      val sub = c.expectSubscription()
+      sub.request(10)
+      for (elem ← List("a", "c")) c.expectNext(elem)
+      c.expectComplete()
+    }
+
     "should handle cancel properly" in {
       val pub = StreamTestKit.PublisherProbe[Int]()
       val sub = StreamTestKit.SubscriberProbe[Int]()