The design for the memory tracking project SPM-3716 places a memory tracker on a query's cursor so that memory stats can be transferred to subsequent getMore requests. This scheme is overly complex and prone to bugs.

Instead we should use the ExpressionContext for this purpose. There will be just one ExpressionContext for the outermost pipeline that lives the entire lifetime of the query, through its getMore invocations.

For stages that are created in sub-pipelines, we need to be able to report to OperationMemoryUsageTracker in the outermost ExpressionContext. I don't believe there is currently a way to get at it when we are constructing a sub-pipeline. This would need to be addressed.