Currently accumulatePipelinePlanSummaryStats() is a free function declared and defined in document_source.{h,cpp}, but
- it can actually (and subjectively more naturally) be a method of a Pipiline class
- it deals with query execution statistics, which will be moved out of DocumentSource classes as a result of SPM-4106
- converting it first to the method of the Pipeline class will simplify further refactoring.