Fix zonedb reload related leaks

Test for and repair a memory leak that caused by reuse_member_zone().

src/knot/zone/zonedb-load.c

@@ -304,7 +304,8 @@ static zone_t *reuse_member_zone(zone_t *zone, server_t *server, conf_t *conf,
 		conf_activate_modules(conf, server, newzone->name, &newzone->query_modules,
 		                      &newzone->query_plan);
 	}
-	zone->change_type |= CONF_IO_TRELOAD;
+	zone->change_type |= CONF_IO_TUNSET;
+	zone->contents = NULL;
 
 	return newzone;
 }