RFR: DRAFT: Expand old on demand [v9]

[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:

  Remove infrastructure to adjust generation capacities for loans from old
  
  A previous mechanism would loan memory from old to young during the
  evacuation and update-refs phases of GC.  This approach had limited
  value because the loans had to be repaid immediately upon completion of
  the update-refs phase.  A newer approach dynamically adjusts the size of
  old generation, consistently keeping it as small as possible.  This
  allows the maximum amount of allocation to be performed between
  consecutive young-gc efforts.

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/248/files
  - new: https://git.openjdk.org/shenandoah/pull/248/files/18217b4c..fc85f085

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=248&range=08
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=248&range=07-08

  Stats: 122 lines in 11 files changed: 2 ins; 105 del; 15 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