SplittableRandom update

[Doug Lea]

In the course of writing up a report (coming soon) that includes
discussion of SplittableRandom, we had a chance to further
analyze and test things, resulting in a few small improvements. Plus
some internal renamings to better reflect intent.
Plus now with the same initial seed mechanics
discussed a few days ago for ThreadLocalRandom.

Thanks to Paul Sandoz for creating webrevs:

https://bugs.openjdk.java.net/browse/JDK-8025136

http://cr.openjdk.java.net/~psandoz/tl/JDK-8025136-SR-enhancements/webrev/

I think we need one more reviewer for it.

-Doug