RFR: jsr166 jdk9 integration wave 4

[Martin Buchholz]

After much debate on what to do when CompleteableFuture.whenComplete
encounters an exception in both the source and the action, we chose
what was acceptable to the most people - add the action's exception to
the source exception as a suppressed exception.  And added usage
guidelines.  And gave handle "top billing" over whenComplete.

http://cr.openjdk.java.net/~martin/webrevs/openjdk9/jsr166-jdk9-integration/

and the usual miscellaneous improvements.