Add last_gtid to session_track_system_variables automatically when causal_reads is enabled

Instead of relying on the DBA to configure it globally for every server, MaxScale should modify the session-level setting when starting a ReadWriteSplit session.

Original description:
Hi

when we enable causal_read on maxscale, even if set, the databases need to have 'session_track_system_variables' parameter must include 'last_gtid''

The variable session_track_system_variables is a dynamic variable and maxscale is able to it by itself , monitor user/ service user has already the grant to be able to do that.

Please set session_track_system_variables include 'last_gtid' by maxscale, if causal_read get enable