Provide optional query execution time in qlafilter log

MaxScale's qlafilter as of version 2.1 allows to optionally include several useful columns in the qlafilter log using log_data parameter. Please, add column for either query end timestamp or query execution time as it is seen by MaxScale.

This will allow to use this filter to create a slow query log of a kind, and to aggregate data to find out the total time spent on queries logged per user or any other filter.