Allow dirty reads/writes

As of today Maxscale doesn't care for gcs.ignore_sb and wsrep_ditry_reads settings.

This means in many cases Maxscale becomes an issue for tpopologies since client can only have 2 states : working and incident and nothing in between where apps can keep reading datas in case of an incident. this is more than usefull in settings where we want to minimize downtime on reads to the very strict minimum.