[jdk17u-dev] RFR: 8227060: Optimize safepoint cleanup subtask order

[Aleksey Shipilev]

Unclean backport to improve the safepoint cleanup times. Provides the grounds for the ultimate improvement in safepoint cleanup ([JDK-8306738](https://bugs.openjdk.org/browse/JDK-8306738)). The backport is not clean, because there are copyright years conflicts, the rename of `AbstractGangTask` to `WorkerTask` is missing, `WorkGang` -> `WorkerThreads` rename is missing, `JavaThread::cast` cleanup is missing.

Additional testing:
 - [x] Ad-hoc benchmarks
 - [x] Linux x86_64 fastdebug `tier1 tier2 tier3`

The safepoint cleanup times are slightly better on small tests, and there seem to be no regressions.
![plot](https://github.com/openjdk/jdk17u-dev/assets/1858943/7e012482-71f8-459f-aca0-4c043b791e92)

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Commit messages:
 - Backport 96a542feb2064dba155ebf05311752995d164038

Changes: https://git.openjdk.org/jdk17u-dev/pull/1377/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=1377&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8227060
  Stats: 62 lines in 5 files changed: 22 ins; 34 del; 6 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/1377.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/1377/head:pull/1377

PR: https://git.openjdk.org/jdk17u-dev/pull/1377