GenShen generally tries to keep the young generation as large as possible. Although the user may specify `ShenandoahMinYoungPercentage` and `ShenandoahMaxYoungPercentage`, GenShen may violate these constraints when promoting regions in place. We believe generation sizing decisions are best left to heuristics. These two options will be combined into a `ShenandoahInitYoungPercentage` to be used to set the initial size for the young generation.