* Fix incorrect materialized types = Unit in stream documentation (Fixes#21937)
* Correct Int, which should be Integer in Java
* Replace BoxedUnit in stream doc
This entails:
* adding akka.pattern.PatternCS.* to enable ask etc. with
CompletionStage
* changing RequestContext to offer an ExecutionContextExecutor for the
CompletionStage.*Async combinators
* splitting up akka.stream.Queue for JavaDSL consistency