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.
Just to make sure I understand it correctly: Is it functionally important to execute both statements in the same query? `LAST_INSERT_ID()` should be stored for the client (=for this PDO connection) by the first statement, so we could split up the query in two (`UPDATE`/`SELECT`), and we would still retrieve it correctly with the second query. This would make the code a bit more readable, and we could remove the `nextRowset()` method altogether.

The downside would be a (very small) performance loss due to the roundtrip and query queuing on the server, and I guess that we could have inconsistencies arising in the very special edge case where the PDO connection is being lost between both queries.