RFR: DRAFT: Expand old on demand [v32]

[Kelvin Nilsen]

> This PR describes several proposed changes to dynamically adjust the sizes of old-gen and young-gen.  In general, the objective is to keep old-gen as small as possible so that there is an abundance of memory available for the young-gen allocation runway.
> 
> As currently drafted, there are regression failures.  This DRAFT PR is published for the purpose of facilitating a careful code review.

Kelvin Nilsen has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains 114 commits:

 - Merge remote-tracking branch 'origin' into expand-old-on-demand
 - Tidy up some comments
 - Remove iteator over mutator threads as not needed
 - Fix a comment re: max_young_evacuation_reserve
 - No need to distinguish generational full gc verification
 - Fix ShouldNotReachHere() assert
 - Remove unnecessary conditional compile directives
 - Remove unnecessary assert
   
   The assert was added at some point during development, but found not to be relevant.
 - Better code reuse for clearing old-gen triggers
 - Tidy up loop for selection of mixed-evacuation candidate regions
 - ... and 104 more: https://git.openjdk.org/shenandoah/compare/21982fc5...42bb8f41

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Changes: https://git.openjdk.org/shenandoah/pull/248/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=248&range=31
  Stats: 3099 lines in 35 files changed: 1692 ins; 884 del; 523 mod
  Patch: https://git.openjdk.org/shenandoah/pull/248.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/248/head:pull/248

PR: https://git.openjdk.org/shenandoah/pull/248