Pipeline::dispose() calls DocumentSource::dispose() for containing DocumentSource subclasses. This name is somewhat confusing. We can follow more intuitive naming and introduce Stage::open() and Stage::close().

In addition, we should move/rename Pipeline::dispose() method to exec::agg::Pipeline::close() and use standard RAII concepts (without a custom deleter).

We should also try to remove DocumentSourceDeleter which is used in aggregation tests.