The PlanExecutor interface currently has the following pure virtual methods for retrieving the next single result:

    virtual ExecState getNext(BSONObj* out, RecordId* dlOut) = 0;
    virtual ExecState getNextDocument(Document* objOut, RecordId* dlOut) = 0;

It turns out that getNextDocument() is never called with a non-nullptr RecordId* in any of PlanExecutor 's sub-classes, so we should remove the dlOut parameter from the getNextDocument() signature entirely.

It also turns out that in many cases the getNext() function internally calls getNextDocument(), with the objOut* pointer being a non-nullptr. We can optimize these cases too.
Streamlining this code will lead to less conditional branches and less temporary object creations on the executor hot path code.