RFR: JDK-8319662 ForkJoinPool trims worker threads too slowly [v7]

[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 eight additional commits since the last revision:

 - Merge branch 'openjdk:master' into JDK-8319662
 - Reduce oversignalling and contention; add test
 - Revert 2 lines in method scan
 - Merge branch 'openjdk:master' into JDK-8319662
 - Split up method awaitWork; other associated changes.
 - Merge branch 'openjdk:master' into JDK-8319662
 - tweak cascades; reinstate an @Contended; resolve JDK-8319498
 - Support cascading idle timeouts

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Changes:
  - all: https://git.openjdk.org/jdk/pull/16725/files
  - new: https://git.openjdk.org/jdk/pull/16725/files/f6a51eb0..6f4b9bb3

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16725&range=06
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16725&range=05-06

  Stats: 17186 lines in 652 files changed: 12488 ins; 2879 del; 1819 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