Replace remaining uses of deprecated db_num_rows() function

The db_num_rows() function is deprecated with the new database API
because PDO doesn't know how many rows are being returned until all rows
have been requested from the server. We therefore have to perform the
row count ourselves - when necessary.

In many cases it may make more sense to perform a SELECT(*) query to
retrieve counts. This is less costly than returning data that we're not
going to use. This is something to consider in the future as the core is
upgraded to use more modern/robust database queries.