MXS-2219 Use system.membership as primary table

From system.membership we can find out what server exist in the
cluster while system.nodeinfo contains information about those
servers. If a node goes down, it will disappear from system.nodeinfo,
but not from system.membership. Consequently, we must start from
system.membership and then fetch more information from system.nodeinfo.

Incidentally, a query like

    SELECT ms.nid, ni.iface_ip
    FROM system.membership AS ms
        LEFT JOIN system.nodeinfo AS ni ON ms.nid=ni.nodeid;

should provide all information in one go, but it seems that such joins
are not supported on the system tables.

server/modules/monitor/clustrixmon/clustrix.hh

@@ -0,0 +1,40 @@
+/*
+ * 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: 2022-01-01
+ *
+ * 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.
+ */
+#pragma once
+
+#include "clustrixmon.hh"
+#include <string>
+
+namespace Clustrix
+{
+
+enum class Status
+{
+    QUORUM,
+    STATIC,
+    UNKNOWN
+};
+
+Status      status_from_string(const std::string& status);
+std::string to_string(Status status);
+
+enum class SubState
+{
+    NORMAL,
+    UNKNOWN
+};
+
+SubState    substate_from_string(const std::string& substate);
+std::string to_string(SubState sub_state);
+
+}