RFR: JDK-8319662 ForkJoinPool trims worker threads too slowly

[Doug Lea]

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

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Commit messages:
 - Support cascading idle timeouts

Changes: https://git.openjdk.org/jdk/pull/16725/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16725&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8319662
  Stats: 63 lines in 1 file changed: 6 ins; 7 del; 50 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