[8u] RFR (S) 8190974: Parallel stream execution within a custom ForkJoinPool should obey the parallelism

[Aleksey Shipilev]

Original fix:
  https://bugs.openjdk.java.net/browse/JDK-8190974
  http://hg.openjdk.java.net/jdk/jdk/rev/e0041b182e31

This is one of the things that was mentioned multiple times as the pitfall in 8u. The patch itself
is simple enough to consider backporting and fixing that pothole. Once related JDK-8154387 [1]
backport is in, this patch applies with reshuffling to 8u, except for the test, which requires
fitting to the old shape of Stream tests.

8u webrev:
  http://cr.openjdk.java.net/~shade/8190974/webrev.8u.01/

Testing: jdk_core tests, including the new test

Thanks,
-Aleksey

[1] https://bugs.openjdk.java.net/browse/JDK-8154387