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

[Albert Mingkun Yang]

> 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

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/23209/files
  - new: https://git.openjdk.org/jdk/pull/23209/files/b660dd79..13921a7d

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=23209&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=23209&range=02-03

  Stats: 9 lines in 5 files changed: 0 ins; 4 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/23209.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23209/head:pull/23209

PR: https://git.openjdk.org/jdk/pull/23209