RFR: DRAFT: Expand old on demand [v2]

[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 one additional commit since the last revision:

  Address a regression in processing of preselected regions
  
  All preselected regions are supposed to be added to the collection set.
  However, an error during implementation of expand-old-on-demand branch
  allowed certain preselected regions not to be placed into the collection
  set.  This has been corrected.  Some comments have also been improved.

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

Changes:
  - all: https://git.openjdk.org/shenandoah/pull/248/files
  - new: https://git.openjdk.org/shenandoah/pull/248/files/40ba78b4..da2de284

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=248&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=248&range=00-01

  Stats: 38 lines in 2 files changed: 10 ins; 8 del; 20 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