RFR: 8327000: GenShen: Integrate updated Shenandoah implementation of FreeSet into GenShen [v8]

[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:

  Minor refinements to test programs
  
  TestAllocIntArrays: comments to explain behavior.
  TestOldGrowthTriggers: reduce the number of loop iterations so this test
  will not time out on less powerful test platforms.

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

Changes:
  - all: https://git.openjdk.org/shenandoah/pull/440/files
  - new: https://git.openjdk.org/shenandoah/pull/440/files/f50528b3..0350ce08

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=440&range=07
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=440&range=06-07

  Stats: 8 lines in 2 files changed: 5 ins; 0 del; 3 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