GenShen: Be less aggressive with triggers for old-gen has expanded

Be default, we trigger an old-gen collection each time the old generation has expanded by more than 12.5%. This works fairly well for many workload, but triggers too aggressively on others. When we trigger too aggressively, we spend too much effort in unproductive old-gen collections, negatively impacting performance.

Suggested improvements:
1. Do not trigger old-gen collections if the total size of old-gen is less than a particular percentage of the full heap size.
2. Make the growth threshold at which we trigger old-gen collections a configuration parameter.