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.cc

@@ -15,3 +15,5 @@
 
 /** Server id default value */
 const int64_t SERVER_ID_UNKNOWN = -1;
+/** Default gtid domain */
+const int64_t GTID_DOMAIN_UNKNOWN = -1;