Simplify Sql.query_row_col function

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.

src/token.rs

@@ -39,7 +39,6 @@ pub fn lookup(context: &Context, namespace: Namespace, foreign_id: u32) -> Optio
         context,
         "SELECT token FROM tokens WHERE namespc=? AND foreign_id=?;",
         params![namespace, foreign_id as i32],
-        0,
     )
 }