RFR: 8325807: Shenandoah: Refactor full gc in preparation for generational mode changes [v2]

[William Kemper]

> Changes to prepare for generational mode:
> * A `phase5_epilogue` method is added to run the final steps of the gc
> * Prepare for mark operation is run by multiple worker threads
> * `finish_region` method of compacting preparation closure is renamed to `finish`
> * The prepare for compaction loop is extracted to a template method, parameterized on closure type

William Kemper 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 'jdk/master' into prepare-isolate-full-gc
 - Factor out epilogue and template function to prepare for compaction

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Changes:
  - all: https://git.openjdk.org/jdk/pull/17894/files
  - new: https://git.openjdk.org/jdk/pull/17894/files/c631bde4..afc10227

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17894&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17894&range=00-01

  Stats: 33400 lines in 1435 files changed: 14700 ins; 12499 del; 6201 mod
  Patch: https://git.openjdk.org/jdk/pull/17894.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17894/head:pull/17894

PR: https://git.openjdk.org/jdk/pull/17894