Add query dispatcher to allow query modification

The goal here is to allow queries to be modified at a very low level - in this use case to prepend comment data as to the user that triggered the query for forensic purposes per

https://github.com/eileenmcnaughton/org.wikimedia.userquerydata/blob/master/userquerydata.php#L10

In discussion with Tim Otten it seemed the performance overhead would be very low where there is no event added (and with the event too - dependent on the code in the event).