RFR: JDK-8319662 ForkJoinPool trims worker threads too slowly [v3]

[Doug Lea]

> This update cascades timeouts to trim subsequent workers after the first  keepAlive inactive period.

Doug Lea 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 three additional commits since the last revision:

 - Merge branch 'openjdk:master' into JDK-8319662
 - tweak cascades; reinstate an @Contended; resolve JDK-8319498
 - Support cascading idle timeouts

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/16725/files
  - new: https://git.openjdk.org/jdk/pull/16725/files/a3c96c6a..6d352608

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16725&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16725&range=01-02

  Stats: 6770 lines in 217 files changed: 3887 ins; 868 del; 2015 mod
  Patch: https://git.openjdk.org/jdk/pull/16725.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16725/head:pull/16725

PR: https://git.openjdk.org/jdk/pull/16725