Support GET_LOCK and friends.

In a two-server setup (one primary and one replica) with two MaxScale instances, a third node is needed as an arbitrator so that the monitors in respective MaxScales can reach an agreement, who runs the show. One alternative for that arbitrator is to install a MariaDB server on the host where one of the MaxScales is running. That MariaDB server is not used for anything "real", but only as an arbitrator.

An alternative for would be to create a MaxScale router that implements the SQL commands and only the SQL commands needed by the monitors (GET_LOCK and friends) . That way there would be no need to install a MariaDB server on the MaxScale host.