RFR: DRAFT: Expand old on demand [v34]

[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:

  Tidy up after merge from mainline
  
  Some incompatibilities between the different development branches required
  relaxing assertions that only generational mode accounts for usage
  within generations.  Also, we needed a fix in the handling of regions
  promoted in place when they are made free (only if they have a
  sufficiently large alloc_capacity.

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

Changes:
  - all: https://git.openjdk.org/shenandoah/pull/248/files
  - new: https://git.openjdk.org/shenandoah/pull/248/files/30415b40..8c96288b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=248&range=33
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=248&range=32-33

  Stats: 12 lines in 2 files changed: 2 ins; 9 del; 1 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