RFR: 8322255: Generational ZGC: Improve heuristics about page size

[Guoxiong Li]

Hi all,

This patch fixes several issues about heuristics.

1. Move `ZHeuristics::set_medium_page_size` to `ZArguments::initialize` so that the setting of `MaxTenuringThreshold` can use `ZPageSizeMedium` correctly.
2. Fix `ZHeuristics::use_per_cpu_shared_small_pages` to meet its comment.
3. Fix `ZHeuristics::significant_heap_overhead` and `ZHeuristics::significant_young_overhead` to use `ZFragmentationLimit` and `ZYoungCompactionLimit` as percentages instead of multiples.

Thanks for the review.

Best Regards,
-- Guoxiong

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Commit messages:
 - JDK-8322255

Changes: https://git.openjdk.org/jdk/pull/17137/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17137&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8322255
  Stats: 8 lines in 3 files changed: 3 ins; 2 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/17137.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17137/head:pull/17137

PR: https://git.openjdk.org/jdk/pull/17137