RFR: 8325673: GenShen: Share Reserves between Old and Young Collector [v4]

[Kelvin Nilsen]

> Allow young-gen Collector reserve to share memory with old-gen Collector reserve in order to support prompt processing of mixed evacuations, as constrained by ShenandoahOldEvacRatioPercent.

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

 - Simplify arguments by using instance variables in ShenandoahOldHeuristics
 - Use mixed evac rather than piggyback to describe old-gen evacuations

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/395/files
  - new: https://git.openjdk.org/shenandoah/pull/395/files/3e38c8a3..406d347b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=395&range=03
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=395&range=02-03

  Stats: 447 lines in 4 files changed: 51 ins; 310 del; 86 mod
  Patch: https://git.openjdk.org/shenandoah/pull/395.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/395/head:pull/395

PR: https://git.openjdk.org/shenandoah/pull/395