MXS-1778: Add support for MariaDB GTID tracking

The MariaDB implementation allows the last GTID to be tracked with the
`last_gtid` variable. To do this, the configuration option
`session_track_system_variables=last_gtid` must be used or it must be
enabled at runtime.

include/maxscale/protocol/mysql.h

@@ -314,6 +314,12 @@ typedef enum
 
 static const mxs_mysql_cmd_t MXS_COM_UNDEFINED = (mxs_mysql_cmd_t) - 1;
 
+/**
+ * A GWBUF property with this name will contain the latest GTID in string form.
+ * This information is only available in OK packets.
+ */
+static const char* const MXS_LAST_GTID = "last_gtid";
+
 /**
  * List of server commands, and number of response packets are stored here.
  * server_command_t is used in MySQLProtocol structure, so for each DCB there is