Function `query_row` executes query and calls callback function to process
row returned. Function query_row_col() is special case, that provides
callback function, which returns value of one particular column of row,
ignoring others.
In all cases, that particular column was 0 (first and only column of
query result), since there is no point to select more than one column
with this function -- they are discarded anyway.
This commit removes that redundancy, removing column number argument of
query_row_col() function and adjusting call sites accordingly.