RFR: 8325671: Shenandoah: Introduce a ShenandoahGenerationType and templatize certain marking closures with it

[Y. Srinivas Ramakrishna]

8325671: Shenandoah: Introduce a ShenandoahGenerationType and templatize certain marking closures with it

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Commit messages:
 - Merge branch 'master' into generation_type
 - Introduce ShenandoahGenerationType and templatize most closures with it.

Changes: https://git.openjdk.org/jdk/pull/17815/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17815&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325671
  Stats: 119 lines in 9 files changed: 68 ins; 0 del; 51 mod
  Patch: https://git.openjdk.org/jdk/pull/17815.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17815/head:pull/17815

PR: https://git.openjdk.org/jdk/pull/17815