RFR: cli options for young/old ShenandoahMinFreeThreshold [v2]

[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.

Joshua Cao has updated the pull request incrementally with one additional commit since the last revision:

  Update docs for Shenandoah minimum thresholds

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/150/files
  - new: https://git.openjdk.org/shenandoah/pull/150/files/92adffa0..2f7f9eb0

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=150&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=150&range=00-01

  Stats: 9 lines in 1 file changed: 1 ins; 0 del; 8 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