cm_requestClient could use a *counter* instead of storing each id

Incrementing the counter (and getting the new value) needs to be an atomic operation.
Well I understand @fauvel is not only about 1 or 2 sqls. He's rather about mitigating hackiness of this solution. First we use LAST_INSERT_ID in this special, hacky way, then we use this pdo-like nextRowset(). I understand that @fauvel just wanted to make it simpler by using only one of these hacks - which I (personally) agree to. 

I also understand from mysql manual that @fauvel solution will work as described. Except this pdo connection drop or maybe also when single script will fork (with already conected mysql) and send those queries twice from two threads.

I don't like it then as it has this downside. Ideally it would be good to have more friendly way of getting second or all results of Db::exec(). Mapping pdo method is simple, but too pdo-oriented (not abstract enough) way.