RFR: Implement ShenandoahHeap::requires_barriers() [v2]

[Roman Kennke]

> This add proper implementation of requires_barriers() for ShenandoahHeap, and adds some comments to CollectedHeap about it.
> 
> Testing (with a few other fixes in the pipeline):
>  - [x] java/lang/Continuation (-XX:+UseShenandoahGC -XX:ShenandoahGCMode=aggressive)
>  - [x] java/lang/Thread/virtual  (-XX:+UseShenandoahGC -XX:ShenandoahGCMode=aggressive)

Roman Kennke has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains two additional commits since the last revision:

 - Merge remote-tracking branch 'upstream/fibers' into shenandoah-req-barriers
 - Implement ShenandoahHeap::requires_barriers()

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

Changes:
  - all: https://git.openjdk.java.net/loom/pull/29/files
  - new: https://git.openjdk.java.net/loom/pull/29/files/a5e90031..1095d765

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=loom&pr=29&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=loom&pr=29&range=00-01

  Stats: 22255 lines in 713 files changed: 13945 ins; 5604 del; 2706 mod
  Patch: https://git.openjdk.java.net/loom/pull/29.diff
  Fetch: git fetch https://git.openjdk.java.net/loom pull/29/head:pull/29

PR: https://git.openjdk.java.net/loom/pull/29