Implement a query throttling filter

A filter which would implement and improve upon the dbfwfilter's limit_queries rule would be useful for situations where sudden traffic spikes cause a performance loss. This would also improve the quality of service by guaranteeing a consistent rate of queries to the backend instead of large spikes.

The dbfwfilter's limit_queries rule is documented here: https://github.com/mariadb-corporation/MaxScale/blob/2.2/Documentation/Filters/Database-Firewall-Filter.md#limit_queries