RFR: Initial sizing refactor [v2]

[William Kemper]

> Some things to highlight here:
> * This change borrows a bit of code from G1 to handle processing of command line arguments used to size the young generation.
> * A (hard coded for now) threshold on the difference between young/old time has been added to reduce resizing churn.
> * The adaptive heuristic doesn't consider the `soft_tail` anymore. `available` is already adjusted for the soft max capacity.
> * `SoftMaxHeapSize` is used to compute the soft max size and max size for the young generation.

William Kemper has updated the pull request incrementally with one additional commit since the last revision:

  Improve assertions and comments

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/185/files
  - new: https://git.openjdk.org/shenandoah/pull/185/files/193f0975..30caeadc

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=185&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=185&range=00-01

  Stats: 46 lines in 5 files changed: 34 ins; 9 del; 3 mod
  Patch: https://git.openjdk.org/shenandoah/pull/185.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah pull/185/head:pull/185

PR: https://git.openjdk.org/shenandoah/pull/185