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.
But then we will have racing problems when two request will write at the same time. 
This is a suggested use case of MySQL to "misuse" the  LAST_INSERT_ID function.

As you can see I set the LAST_INSERT_ID in the UPDATE to SELECT it in the second query.
see http://dev.mysql.com/doc/refman/5.0/en/information-functions.html#function_last-insert-id
section `If expr is given as an argument to LAST_INSERT_ID()`
This is how it should done. 

@fauvel If you have a better solution which will real work. Feel free to update my pull request.