RFR: 8348171: Refactor GenerationCounters and its subclasses [v4]

[Zhengyu Gu]

On Fri, 31 Jan 2025 12:30:30 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> Simple refactoring of removing the use of `virtual` method and use concrete subclasses when needed.
>> 
>> Test: tier1-5
>
> Albert Mingkun Yang has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - review
>  - * some more refactoring

`Shenandoah` code no longer aligns to others. Other than that, LGTM.

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Marked as reviewed by zgu (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/23209#pullrequestreview-2588364961