RFR: 8325808: GenShen: Move generational mode code out of shFullGC.cpp [v2]

[Kelvin Nilsen]

On Mon, 26 Feb 2024 23:29:18 GMT, William Kemper <wkemper at openjdk.org> wrote:

>> This change reduces the differences from the upstream branch by moving large chunks of generational mode code into separate files.
>
> William Kemper has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Improve names and comments

Thanks for this work and thanks Ramki for thorough review.

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Marked as reviewed by kdnilsen (Committer).

PR Review: https://git.openjdk.org/shenandoah/pull/398#pullrequestreview-1903972471