First sketch of flexible merge
+str #15079 Add flexible merge
Merge pull request #16052 from akka/wip-15079-flexi-merge-patriknw

+str #15079 Add flexible merge
+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