The single-request-in-actor-example does not consume the response
in where the status code is not OK. Although this is unlikely
to cause backpressure issues as the response is likely to have
a HttpEntity.Strict entity, the example is confusing as it
does not follow the warning about always consuming the response.
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
