RFR: 8369913: Parallel: Refactor PSAdjustTask [v2]

[Albert Mingkun Yang]

> Cleanup roots processing (adjusting/remapping) during the adjust-phase of full-gc, so that relevant roots are grouped together with some documentations. Also, the use of `SubTasksDone` is replaced with a `volatile bool`, which is more transparent in the context.
> 
> Test: tier1-5

Albert Mingkun Yang 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 three additional commits since the last revision:

 - review
 - Merge branch 'master' into pgc-adjust-root
 - pgc-adjust-root

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/27822/files
  - new: https://git.openjdk.org/jdk/pull/27822/files/eb0754eb..f5e73b4c

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27822&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27822&range=00-01

  Stats: 70278 lines in 1058 files changed: 38106 ins; 25697 del; 6475 mod
  Patch: https://git.openjdk.org/jdk/pull/27822.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27822/head:pull/27822

PR: https://git.openjdk.org/jdk/pull/27822