Clean up monitor initialization and destruction

Since monitors are now freed at MaxScale exit, the server data should be freed. Also,
gtid domain variables are now initialized with a common constant.

server/modules/monitor/mariadbmon/mariadbmon_common.hh

@@ -33,3 +33,4 @@
     } while (false)
 
 extern const int64_t SERVER_ID_UNKNOWN;
+extern const int64_t GTID_DOMAIN_UNKNOWN;