RFR: 8271514: support JFR use of new ThreadsList::Iterator [v3]

[Daniel D.Daugherty]

> A trivial fix to support JFR use of new ThreadsList::Iterator.
> 
> This fix was tested with Mach5 Tier[1-3].

Daniel D. Daugherty has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 12 commits:

 - Merge branch 'master' into JDK-8271514
 - Merge branch 'pull/4948' into JDK-8271514
 - Merge branch 'master' into JDK-8271513
 - Merge branch 'pull/4948' into JDK-8271514
 - Merge branch 'pull/4671' into JDK-8271513
 - kbarrett CR - simplify 'ThreadsList::Iterator::operator!=(Iterator i)'
 - 8271514: support JFR use of new ThreadsList::Iterator
 - 8271513: support JavaThreadIteratorWithHandle replacement by new ThreadsList::Iterator
 - 8193559.kbarrett.part1 - Kim's proposed rewrite using newer C++ features.
 - Merge branch 'master' into JDK-8193559
 - ... and 2 more: https://git.openjdk.java.net/jdk/compare/fa36e334...8873ddd5

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Changes: https://git.openjdk.java.net/jdk/pull/4949/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4949&range=02
  Stats: 39 lines in 2 files changed: 20 ins; 11 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4949.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4949/head:pull/4949

PR: https://git.openjdk.java.net/jdk/pull/4949