[jdk17u-dev] RFR: 8273559: Shenandoah: Shenandoah should support multi-threaded heap dump

[William Kemper]

Unclean backport of JDK-8273559

Testing:

- [ ] hotspot_gc_shenandoah
- [ ] dacapo
- [ ] extremem
- [ ] specjbb2015
- [ ] hyperalloc

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Commit messages:
 - Backport 8132bfd23f2f7fb52e502a3e6fe488fbdb537df0

Changes: https://git.openjdk.org/jdk17u-dev/pull/1450/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=1450&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8273559
  Stats: 205 lines in 6 files changed: 161 ins; 10 del; 34 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/1450.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/1450/head:pull/1450

PR: https://git.openjdk.org/jdk17u-dev/pull/1450