PlanStage::_commonStats generally does not have to be accessed or modified by stage implementations, and thus should be made private.

There are a couple of existing uses that would have to be addressed before this could be made possible:

• Stages are generally responsible for setting _commonStats.isEOF.
• MultiPlanStage::pickBestPlan(), CachedPlanStage::pickBestPlan() and SubplanStage::pickBestPlan() are responsible for updating _commonStats.executionTimeMillis.