RFR: JDK-8319662 ForkJoinPool trims worker threads too slowly [v6]

[Doug Lea]

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

Doug Lea has updated the pull request incrementally with one additional commit since the last revision:

  Reduce oversignalling and contention; add test

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Changes:
  - all: https://git.openjdk.org/jdk/pull/16725/files
  - new: https://git.openjdk.org/jdk/pull/16725/files/e111166f..f6a51eb0

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16725&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16725&range=04-05

  Stats: 199 lines in 2 files changed: 65 ins; 47 del; 87 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