RFR: DRAFT: Expand old on demand [v16]

[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:

  Do not call min_threshold unless generation is not old
  
  Following the merge, min_threshold() asserts generation is not old.
  min_threshold() is not relevant to old-generation GC triggers.

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/248/files
  - new: https://git.openjdk.org/shenandoah/pull/248/files/4bb56495..157d42d4

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=248&range=15
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=248&range=14-15

  Stats: 4 lines in 1 file changed: 2 ins; 2 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