RFR: 8328220: GenShen: Move generational mode operational parameters out of ShenandoahHeap [v2]

[William Kemper]

> * Many fields previously defined in `ShenandoahHeap` have been moved to `ShenandoahOldGeneration`.
> * Generational specific serviceability has been moved to `ShenandoahGenerationalHeap`.
> * Methods for sizing the old generation have been moved to `ShenandoahGenerationalHeap`.
> * `ShenandoahGenerationalHeap::heap()` now asserts that the generational mode is active.

William Kemper has updated the pull request incrementally with one additional commit since the last revision:

  Improve comments, make more variables const.

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/406/files
  - new: https://git.openjdk.org/shenandoah/pull/406/files/00f8778b..375f1434

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=406&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=406&range=00-01

  Stats: 55 lines in 4 files changed: 4 ins; 9 del; 42 mod
  Patch: https://git.openjdk.org/shenandoah/pull/406.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/406/head:pull/406

PR: https://git.openjdk.org/shenandoah/pull/406