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
Any previous usage can be represented in this fashion while typically
saving one function allocation and also saving the function literal’s
syntax overhead (for the seed). Plus a new feature: the provided
iterator does not have to be infinite, limiting how far expand is
allowed to go.
