Consistent reads by the utlization of GTID in OK packet.

https://jira.mariadb.org/browse/MDEV-11956 will provide the last GTID in the OK packet. That can be utilized for providing consistent reads in the context of MaxScale and readwritesplit.

When e.g. an UPDATE is performed on the master, MaxScale will store the GTID. When a SELECT comes along, MaxScale could inject SELECT MASTER_GTID_WAIT(xyz) before the actual SELECT, thus ensuring that whatever modifications the UPDATE caused, will be available when the actual SELECT is executed on the chosen slave.