Added data field to db_res_t to allow db backends to store auxiliary data

along with the result.

db/db_res.h

@@ -43,6 +43,7 @@ typedef struct db_res {
 	} col;
 	struct db_row* rows;       /* Rows */
 	int n;                     /* Number of rows */
+	void* data;                /* Auxiliary data */
 } db_res_t;