MXS-1703 Use a common header for all mariadbmon files

The mariadb_common.hh file is now a header with only common definitions
such as module name. The QueryResult-class was moved to MariaDBServer.

server/modules/monitor/mariadbmon/mariadbmon_common.cc

@@ -0,0 +1,17 @@
+/*
+ * Copyright (c) 2018 MariaDB Corporation Ab
+ *
+ * Use of this software is governed by the Business Source License included
+ * in the LICENSE.TXT file and at www.mariadb.com/bsl11.
+ *
+ * Change Date: 2020-01-01
+ *
+ * On the date above, in accordance with the Business Source License, use
+ * of this software will be governed by version 2 or later of the General
+ * Public License.
+ */
+
+#include "mariadbmon_common.hh"
+
+/** Server id default value */
+const int64_t SERVER_ID_UNKNOWN = -1;