RFR: 8010151 nsk/regression/b6653214 fails "assert(snapshot != NULL) failed: Worker should not be started"

[Zhengyu Gu]

Second round.

It fixed a racing condition when NMT is being shutdown while worker thread just started. The race causes get_snapshot() to return NULL, that triggers the assertion.

It also fixed a few volatile pointers that were mistakenly declared as volatile objects.

Bug:       http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=8010151
Webrev: http://cr.openjdk.java.net/~zgu/8010151/webrev.01/

Tests:
  - JPRT
  - vm.quick.testlist on Linux x86 and Windows x64
  - The approach described by Christian in bug report. I were able to reproduce on prior the fix, and it is no longer reproducible after.



Thanks,

-Zhengyu