!htc #15726: Akka HTTP with the new DSL
Merge pull request #16049 from drewhk/wip-15716-akka-http-dsl2

!htc #15716: Akka HTTP with the new DSL
+str #15736 Initial version of JavaDSL for akka-streams (NOT complete)

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
+str #15736 Added JavaDSl for PartialFlowGraph
+ enabled more FlowTest tests
+ fixed naming of internal classes (no Java in class names)
+ added converters for UndefinedSink/Source
+str #15736 Extended DslConsistencySpec, minor cleanups