MXS-1533: Better handling of multi-domain gtids

If the gtid_domain_pos of the master is ever modified,
gtid-variables will have multiple domains. Generally, we are
only interested in the most recent domain. This is tracked in
gtid_domain_id:s and the value of the master is used for
filtering the correct domain from all gtid-values.

Also, use gtid_current_pos instead of gtid_slave_pos. The
advantage of current_pos is that the same variable works also
for master servers. The gtid-handling is now more thorough and
detects some weird situations.

server/modules/monitor/mysqlmon.h

@@ -71,6 +71,7 @@ typedef struct
     char* replication_password;    /**< Replication password for failover*/
     bool verify_master_failure;    /**< Whether master failure is verified via slaves */
     int master_failure_timeout;    /**< Time in seconds to wait before doing failover */
+    int64_t master_gtid_domain;    /**< Gtid domain currently used by the master */
     MXS_MONITOR* monitor;
 } MYSQL_MONITOR;