RFR: 8318190: GenShen: Be less aggressive with triggers for old-gen has expanded

[Kelvin Nilsen]

This patch reduces the frequency of old-has-grown triggers in the case that old usage is smaller than a particular threshold percentage (default value 5%) of the total heap size.  In this case, old is triggered for growth no more frequently than once every N cycles, where N defaults to 50.

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Commit messages:
 - Restrict old triggers by size and frequency

Changes: https://git.openjdk.org/shenandoah/pull/341/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=341&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8318190
  Stats: 52 lines in 5 files changed: 48 ins; 2 del; 2 mod
  Patch: https://git.openjdk.org/shenandoah/pull/341.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/341/head:pull/341

PR: https://git.openjdk.org/shenandoah/pull/341