Document Java 9 new CompletableFuture features in Async

[billoneil]

Looks like Java 9 supports overriding the default executor in CompletableFuture. https://docs.oracle.com/javase/9/docs/api/java/util/concurrent/CompletableFuture.html#defaultExecutor--

This could simplify the implementation of SameExecutorCompletionStage in the future but not until support for java8 is dropped.

Does this kill the need for this class entirely? It would be best to remove it before the release if so. I think it might be nice to have for Java 8 though.

[lukaseder]

Hmm, interesting discovery, thanks for sharing. I hadn't looked at these Java 9 improvements yet.

I think it's definitely worth mentioning in the Async Javadoc, but these improvements are only valid for CompletableFuture (not CompletionStage), whose design I still don't fully agree with. So I think we can keep the implementation you've added, given that Java 9 will take quite a while to be adopted.

I'll rename this issue to be the task about adding a comment to the Javadoc...