RFR: DRAFT: Expand old on demand [v39]

[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:

  Reduce guaranteed young GC interval to 30 seconds
  
  It had been set to 15 seconds after observing that the default value of
  5 minutes was resulting in degenerated cycles because GC triggers were
  too lazy.  The value of 30 seconds has been observed to not result in
  degenerated GC with our two PhasedUpdates netflix workloads.

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/248/files
  - new: https://git.openjdk.org/shenandoah/pull/248/files/b0b61a45..ba5616d5

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=248&range=38
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=248&range=37-38

  Stats: 1 line in 1 file changed: 0 ins; 0 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