class DocumentSource currently still virtually inherits from RefCountable here:

// TODO SPM-4106: Remove virtual keyword once the refactoring is done.
class DocumentSource : public virtual RefCountable {

The virtual inhertiance was necessary during the SPM-4106 transition period, when the individual DocumentSource subclasses inherited from both DocumentSource and exec::agg::Stage.
Now that no DocumentSource subclass inherits from exec::agg::Stage anymore, we should remove the virtual inheritance and the TODO from the code.