RFR: 8279294: NonblockingQueue::try_pop may improperly indicate queue is empty

[Kim Barrett]

Please review this improvement to NonblockingQueue::try_pop.  The old code
returned an indication that the queue was empty in some cases where that
wasn't true.  In particular, contending try_pop operations could result in
some incorrectly indicating empty.  The change fixes that and improves the
interaction between contending try_pops.

Testing:
mach5 tier1-3

Lots of testing of this change in conjunction with others as part of
investigating and fixing JDK-8273383.

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Commit messages:
 - fix

Changes: https://git.openjdk.java.net/jdk/pull/7149/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7149&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8279294
  Stats: 44 lines in 1 file changed: 23 ins; 3 del; 18 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7149.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7149/head:pull/7149

PR: https://git.openjdk.java.net/jdk/pull/7149