RFA 8056249 Improve CompletableFuture resource usage

[Paul Sandoz]

Hi,

I would like to backport the improvements of CompletableFuture from 9 to 8u40:

  http://mail.openjdk.java.net/pipermail/core-libs-dev/2014-August/028418.html

  https://bugs.openjdk.java.net/browse/JDK-8056249
  
  http://cr.openjdk.java.net/~psandoz/jdk9/JDK-8056249-cf-resource-usage/webrev/

  http://hg.openjdk.java.net/jdk9/dev/jdk/rev/68a6bb51cb26

The changeset (when shuffled) imports cleanly into jdk8u-dev/jdk.

This fix exposed an issue with some JCK tests relying on implementation behaviour that changed [*]. JCK 8a has been updated accordingly.

Paul.

[*] Regarding a stage that is dependent of either of two other stages and one completes exceptionally. On CompletionStage:

 * to either one of these exceptions.  If a stage is dependent on
 * <em>either</em> of two others, and only one of them completes
 * exceptionally, no guarantees are made about whether the dependent
 * stage completes normally or exceptionally. In the case of method