RFR 8056249 Improve CompletableFuture resource usage

[Paul Sandoz]

Hi,

Please review fixes by Doug to j.u.c.CompletableFuture to better control resources for long completion chains (e.g. avoiding stack overflows). This fix resulted in a lot of internal refactoring and clean up.  There are also some doc clarifications for certain j.u.c.CompletationStage exception handling methods (which most likely means a CCC is required).

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


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Is the following snippet missing from the doc updates to the methods CompletationStage.handle and handleAsync?

  If the supplied function itself encounters an
  exception, then the returned stage exceptionally completes with this
  exception unless this stage also completed exceptionally.

Paul.