RFR: DRAFT: Expand old on demand [v26]

[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 incrementally with two additional commits since the last revision:

 - Tidy up loop for selection of mixed-evacuation candidate regions
 - Fix loop that selects old-gen candidates for subsequent mixed evac

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/248/files
  - new: https://git.openjdk.org/shenandoah/pull/248/files/f13b55ba..c485c006

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=248&range=25
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=248&range=24-25

  Stats: 9 lines in 1 file changed: 2 ins; 1 del; 6 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