RFR 8027316 Distinct operation on an unordered stream should not be a barrier

[Paul Sandoz]

Hi

Please review:

  http://cr.openjdk.java.net/~psandoz/tl/JDK-8027316-distinct-unordered/webrev/

The fix ensures that the distinct operation, for a parallel stream, is not a 
full barrier if the upstream is unordered (and not already distinct, otherwise 
it is a no-op).

Thanks,
Paul.