RFR: 8327000: GenShen: Integrate updated Shenandoah implementation of FreeSet into GenShen

[Kelvin Nilsen]

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

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Commit messages:
 - 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
 - Disable debug instrumentation
 - Fix two errors and cleanup some debug instrumentation
 - ... and 46 more: https://git.openjdk.org/shenandoah/compare/6071b4a9...51f4136a

Changes: https://git.openjdk.org/shenandoah/pull/440/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=440&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8327000
  Stats: 6600 lines in 125 files changed: 4580 ins; 962 del; 1058 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