RFR: 8325673: GenShen: Share Reserves between Old and Young Collector

[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.

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Commit messages:
 - Share reserves between Young Collector and Old Collector

Changes: https://git.openjdk.org/shenandoah/pull/395/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=395&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8325673
  Stats: 28 lines in 3 files changed: 15 ins; 0 del; 13 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