RFR: 8327000: GenShen: Integrate updated Shenandoah implementation of FreeSet into GenShen [v19]

[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 five additional commits since the last revision:

 - Shrink iterations again so TestOldGrowth won't timeout on macos-aarch64
 - Improve comments
 - Remove extrameous reserve_regions() declaration
 - Update comments as guided by reviewer requests
 - Adjust locking to reduce frequency of lock acquisition

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

Changes:
  - all: https://git.openjdk.org/shenandoah/pull/440/files
  - new: https://git.openjdk.org/shenandoah/pull/440/files/d2486552..7a0c4817

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=440&range=18
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=440&range=17-18

  Stats: 42 lines in 3 files changed: 9 ins; 15 del; 18 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