mango: communicate rows read for global stats collection

Besides the execution statistics that could be shown for the user,
the number of rows and documents read are tracked separately on
the global level.  (Which is more like a bug than a feature, but
that is out of the scope of this change.)

With the introduction of covering indexes, these two have been
become different, therefore global counting has to be taught about
the number of times when no documents were read but rows only.