[jdk17u-dev] RFR: 8307348: Parallelize heap walk for ObjectCount(AfterGC) JFR event collection

[Aleksey Shipilev]

Improves ObjectCount JFR events that take considerable time at GC pause. While the event is still heavy, the parallelization helps to avoid unnecessarily large stalls. 

The backport is not clean, because there are number of contextual differences: `WorkGang`, `WithUpdatedActiveWorkers` and other renames did not happen yet. I had to massage the patch to fit 17u better.

Additional testing:
 - [x] Linux x86_64 fastdebug `jdk/jfr serviceability/sa`
 - [ ] Linux x86_64 fastdebug `tier1 tier2 tier3`
 - [ ] Linux AArch64 fastdebug `tier1 tier2 tier3`

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Depends on: https://git.openjdk.org/jdk17u-dev/pull/1440

Commit messages:
 - Backport 540c706bbcbb809ae1304aac4f2a16a5e83cb458

Changes: https://git.openjdk.org/jdk17u-dev/pull/1441/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=1441&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307348
  Stats: 41 lines in 9 files changed: 10 ins; 10 del; 21 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/1441.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/1441/head:pull/1441

PR: https://git.openjdk.org/jdk17u-dev/pull/1441