RFR 8005696: Add CompletableFuture - JEP 155

[Chris Hegarty]

As part of JEP 155 (Concurrency Updates) we are proposing to add 
CompletableFuture. A Future that may be explicitly completed (setting 
its value and status), and may include dependent functions and actions 
that trigger upon its completion.

Specdiff:
 
http://cr.openjdk.java.net/~chegar/8005696/specdiff/java/util/concurrent/package-summary.html
Webrev:
   http://cr.openjdk.java.net/~chegar/8005696/webrev.00/webrev/

CompletableFuture, as usual, is coming from Doug Lea, with assistance 
from members of the former JCP JSR-166 Expert Group. I have already 
informally reviewed this. This is an official call for review before 
finalizing the API for inclusion in jdk8.

Note:
   I still need to make some additions to the test for better coverage, 
but these should not be a blocker for this review.

-Chris.