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

[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

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

Commit messages:
 - Factor out epilogue and template function to prepare for compaction

Changes: https://git.openjdk.org/jdk/pull/17894/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17894&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325807
  Stats: 76 lines in 2 files changed: 33 ins; 14 del; 29 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