RFR: Expand old on demand [v56]

[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.
> 
> This PR now passes all GHA pre-integration tests and other internal CI tests.

Kelvin Nilsen has updated the pull request incrementally with two additional commits since the last revision:

 - Fix merge error
 - Move Collector-free regions to Mutator-free at start of update refs
   
   There is no need to maintain a reserve of regions once evacuation has
   completed.  This extends the mutator allocation runway and reduces the
   likelihood of degenerated GC.

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/248/files
  - new: https://git.openjdk.org/shenandoah/pull/248/files/53c899cd..9195007a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=248&range=55
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=248&range=54-55

  Stats: 83 lines in 3 files changed: 79 ins; 1 del; 3 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