* Fold Tap/Drain into Source/Sink
* Create Source/Sink helpers to create all Sources/Sinks
* Make concrete Source/Sink implementations private[scaladsl2]
Missing features are:
+ be able to use all junctions from Java
* re-think akka.stream.javadsl.japi (because variance)
+ better docs
+ enable all tests
+ Partial flow graph as well as GraphFlow tests
+ enabled more FlowTest tests
+ fixed naming of internal classes (no Java in class names)
+ added converters for UndefinedSink/Source