RFR: 8271513: support JavaThreadIteratorWithHandle replacement by new ThreadsList::Iterator [v2]

[Daniel D.Daugherty]

> A trivial fix to support JavaThreadIteratorWithHandle replacement by 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 incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains seven additional commits since the last revision:

 - Merge branch 'pull/4671' into JDK-8271513
 - kbarrett CR - simplify 'ThreadsList::Iterator::operator!=(Iterator i)'
 - 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
 - kbarrett CR - delete unused _list member.
 - 8193559.cr0

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4948/files
  - new: https://git.openjdk.java.net/jdk/pull/4948/files/b3edb5c4..b3edb5c4

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4948&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4948&range=00-01

  Stats: 0 lines in 0 files changed: 0 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4948.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4948/head:pull/4948

PR: https://git.openjdk.java.net/jdk/pull/4948