In addition towards working towards the general goal of
doing as much query execution as possible with a PlanStage
tree, this should have a positive performance impact for
certain agg pipelines. Previously, a pipeline with a
$project (or a $project-like stage such as $addFields)
followed by a $limit might have applied this limit only
after a full batch of data was loaded by
DocumentSourceCursor. After this change, the limit will take
effect prior to DocumentSourceCursor batching, and thus may
reduce the amount of data processed by the query.