RFR: 8327000: GenShen: Integrate updated Shenandoah implementation of FreeSet into GenShen [v27]

[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:

  Improve consistency of OOM in GenShen mode
  
  Treat any full GC that enables a previously blocked allocation as good
  progress.

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

Changes:
  - all: https://git.openjdk.org/shenandoah/pull/440/files
  - new: https://git.openjdk.org/shenandoah/pull/440/files/5400c9c0..91c3ab07

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=440&range=26
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=440&range=25-26

  Stats: 13 lines in 2 files changed: 5 ins; 4 del; 4 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