RFR: 8258284: clean up issues with nested ThreadsListHandles

[Daniel D.Daugherty]

While working on JDK-8231627, I discovered there were issues with nested
ThreadsListHandles. I added a new test to verify how ThreadsListHandles and
nested ThreadsListHandles are supposed to work. I found two bugs:

1) A failure to restore the expected _threads_hazard_ptr when a nested
    ThreadsListHandle is destroyed.

2) A failure to decrement _nested_threads_hazard_ptr_cnt when a nested
    ThreadsListHandle is destroyed.

The second failure mode is what tripped up my fix for JDK-8231627 so that
bug fix depends on this bug fix.

Mach5 Tier[1-3] testing has no regressions (macOSX testing is incomplete at the moment).

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Commit messages:
 - 8258284

Changes: https://git.openjdk.java.net/jdk/pull/1844/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1844&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8258284
  Stats: 720 lines in 4 files changed: 705 ins; 5 del; 10 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1844.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1844/head:pull/1844

PR: https://git.openjdk.java.net/jdk/pull/1844