RFR (M) CR 8023234: StampedLock serializes readers on writer unlock

[Aleksey Shipilev]

Hi,

This is the review for the issue originally found by Dr. Heinz Kabutz
and Victor Grazi. The issue is consequently diagnosed and fixed by Doug
Lea in JSR 166 CVS.

The webrev which brings in the fix and the regression tests in JDK 8:
   http://cr.openjdk.java.net/~shade/8023234/webrev.00/

Testing:
  - builds on Linux x86_64/release
  - targeted regression test failure/pass before/after the fix
  - java/util/concurrent/locks/StampedLock basic tests

Thanks,
-Aleksey.