RFR: 8333750: GenShen: Only instantiate young/old generations in generational mode [v4]

[William Kemper]

> * Other modes should not instantiate or touch young/old generations.
> * The generation sizer has been put in its own file and moved out of shHeap
> * The logic for aging regions has been decoupled from non-generational final update refs

William Kemper has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 38 commits:

 - Simplify affiliation accounting on allocation path
 - Reduce mode checks when making a region young
 - Improve method name and comments
 - Merge remote-tracking branch 'shenandoah/master' into move-young-old-generations
 - Make age and youth const
 - Do not age regions after completing old mark
 - Detect cancellation of old GC after final mark.
 - Fix zero build
 - Add missing precompiled header
 - Fix merge error
 - ... and 28 more: https://git.openjdk.org/shenandoah/compare/e3c3098a...694a4d29

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

Changes: https://git.openjdk.org/shenandoah/pull/444/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=444&range=03
  Stats: 890 lines in 22 files changed: 471 ins; 323 del; 96 mod
  Patch: https://git.openjdk.org/shenandoah/pull/444.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/444/head:pull/444

PR: https://git.openjdk.org/shenandoah/pull/444