The plan cache project is wrapping up so we would like to get rid of featureFlagSbePlanCache. In order to avoid the proliferation of flags, it makes sense to put the SBE plan cache behind featureFlagSbeFull. That way when the SBE performance project enables full SBE by default, the plan cache will get enabled as well. It will also ensure that measurements of full SBE performance incorporate the performance benefits of the SBE plan cache.