RFR: cli options for young/old ShenandoahMinFreeThreshold

[Joshua Cao]

We currently can set only a single `ShenandoahMinFreeThreshold`, which will run a GC cycle if an allocation space fills up to a certain capacity. This PR allows us to specify separate thresholds for young and old generations.

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Commit messages:
 - cli options for young/old ShenandoahMinFreeThreshold

Changes: https://git.openjdk.org/shenandoah/pull/150/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=150&range=00
  Stats: 20 lines in 6 files changed: 17 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/shenandoah/pull/150.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah pull/150/head:pull/150

PR: https://git.openjdk.org/shenandoah/pull/150