Speedup pull request validation

* speedup ActorCreationPerfSpec
* reduce iterations in ConsistencySpec
* tag SupervisorHierarchySpec as LongRunningTest
* various small speedups and tagging in actor-tests
* speedup expectNoMsg in stream-tests
* tag FramingSpec, and reduce iterations
* speedup QueueSourceSpec
* tag some stream-tests
* reduce iterations in persistence.PerformanceSpec
* reduce iterations in some cluster perf tests
* tag RemoteWatcherSpec
* tag InterpreterStressSpec
* remove LongRunning from ClusterConsistentHashingRouterSpec
* sys property to disable multi-jvm tests in test
* actually disable multi-node tests in validatePullRequest
* doc sbt flags in CONTRIBUTING

akka-stream/src/main/scala/akka/stream/scaladsl/Source.scala

@@ -360,7 +360,7 @@ object Source {
    * the materialized future is completed with its value, if downstream cancels or fails without any demand the
    * create factory is never called and the materialized `Future` is failed.
    */
-  def lazily[T, M](create: () => Source[T, M]): Source[T, Future[M]] =
+  def lazily[T, M](create: () ⇒ Source[T, M]): Source[T, Future[M]] =
     Source.fromGraph(new LazySource[T, M](create))
 
   /**