RFR: DRAFT: Expand old on demand [v37]

[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:

 - Reorder lines of code to reduce diffs from original code
 - Remove irrelevant comment
   
   ShenandoahGenerationSizer::heap_size_changed() comment said this was not
   hooked anywhere.  More recently, we have figured out two places that
   need to invoke this service.

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

Changes:
  - all: https://git.openjdk.org/shenandoah/pull/248/files
  - new: https://git.openjdk.org/shenandoah/pull/248/files/cda0e412..b59ece73

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=248&range=36
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=248&range=35-36

  Stats: 40 lines in 2 files changed: 20 ins; 20 del; 0 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