RFR: 8327000: GenShen: Integrate updated Shenandoah implementation of FreeSet into GenShen [v2]

[Kelvin Nilsen]

> The mainline implementation of ShenandoahFreeSet was recently updated.  This PR integrates the upstream changes
> into Generational Shenandoah.

Kelvin Nilsen has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 28 commits:

 - Merge remote-tracking branch 'origin/master' into harmonize-free-set
 - Merge branch 'openjdk:master' into master
 - Remove TestHumongousThreshold test from problem list
 - Fix white space
 - Merge branch 'master' of https://git.openjdk.org/shenandoah into harmonize-free-set
 - Remove debugging instrumentation
 - Merge remote-tracking branch 'origin/master' into harmonize-free-set
 - Merge branch 'openjdk:master' into master
 - Enhance diagnostic message for verification error
 - Some fixes related to TestThreadFailure.java
 - ... and 18 more: https://git.openjdk.org/shenandoah/compare/e3c3098a...bc3cba66

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Changes: https://git.openjdk.org/shenandoah/pull/440/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=440&range=01
  Stats: 1659 lines in 14 files changed: 865 ins; 219 del; 575 mod
  Patch: https://git.openjdk.org/shenandoah/pull/440.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/440/head:pull/440

PR: https://git.openjdk.org/shenandoah/pull/440