RFR: 8318619: GenShen: Add configurable threshold for young triggers to expedite old evacuations

[William Kemper]

When Shenandoah expects to free up space by promotions into young, or evacuations in old it may trigger a young collection. This change sets configurable thresholds for these conditions.

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Commit messages:
 - Can't used checked cast here
 - Create configurable thresholds for expedited triggers

Changes: https://git.openjdk.org/shenandoah/pull/347/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=347&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318619
  Stats: 16 lines in 2 files changed: 13 ins; 1 del; 2 mod
  Patch: https://git.openjdk.org/shenandoah/pull/347.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/347/head:pull/347

PR: https://git.openjdk.org/shenandoah/pull/347