+str - Fixes #16065 by adding an IteratorPublisher and removing SimpleCallbackPublisher

Adds an IteratorPublisher and IteratorPublisherImpl
Removes the SimpleCallbackPublisher
Switches the callback-source () => Option[T] to use a new ThunkIterator and the IteratorPublisher
Removes the use of a ControlThrowable (Stop) for signalling end since we now use Iterator
Improves a couple of streams-related tests so when they fail we get better output

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

@@ -116,7 +116,7 @@ object Source {
    * The stream ends normally when evaluation of the closure returns a `None`.
    * The stream ends exceptionally when an exception is thrown from the closure.
    */
-  def apply[T](f: () ⇒ Option[T]): Source[T] = ThunkSource(f)
+  def apply[T](f: () ⇒ Option[T]): Source[T] = IteratorSource(new ThunkIterator(f))
 
   /**
    * Start a new `Source` from the given `Future`. The stream will consist of