RFR: 8327000: GenShen: Integrate updated Shenandoah implementation of FreeSet into GenShen [v3]

[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 incrementally with one additional commit since the last revision:

  Resolve conflicts introduced by manual resolution of merge

-------------

Changes:
  - all: https://git.openjdk.org/shenandoah/pull/440/files
  - new: https://git.openjdk.org/shenandoah/pull/440/files/bc3cba66..ac9ecfe9

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=440&range=02
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=440&range=01-02

  Stats: 50 lines in 4 files changed: 23 ins; 15 del; 12 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