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

[Y. Srinivas Ramakrishna]

On Fri, 16 Feb 2024 19:49:07 GMT, William Kemper <wkemper at openjdk.org> wrote:

> 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

Marked as reviewed by ysr (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/17894#pullrequestreview-1888700081