RFR: DRAFT: Expand old on demand [v38]

[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 code for integration
  
  Two "global" variables are no longer needed: thread_iteration_roots and
  MIXED_EVACUATIONS_ENABLED.

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

Changes:
  - all: https://git.openjdk.org/shenandoah/pull/248/files
  - new: https://git.openjdk.org/shenandoah/pull/248/files/b59ece73..b0b61a45

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=248&range=37
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=248&range=36-37

  Stats: 9 lines in 3 files changed: 0 ins; 5 del; 4 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