RFR: JDK-8320336: GenShen: Reduce proactive humongous defragmentation efforts

[Kelvin Nilsen]

Only trigger defragmentation of old-gen memory if JVM is configured to reserve a certain amount of the heap to support humongous objects.  Even when the JVM is so configured, be slightly less aggressive in triggering defragmentation of old-gen memory.

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Commit messages:
 - Make old-gen fragmentation triggers less sensitive

Changes: https://git.openjdk.org/shenandoah/pull/361/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=361&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8320336
  Stats: 12 lines in 1 file changed: 4 ins; 1 del; 7 mod
  Patch: https://git.openjdk.org/shenandoah/pull/361.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/361/head:pull/361

PR: https://git.openjdk.org/shenandoah/pull/361