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.cc

@@ -0,0 +1,88 @@
+/*
+ * 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.
+ */
+
+#include "clustrix.hh"
+#include <maxbase/assert.h>
+
+namespace
+{
+
+const char CN_NORMAL[]  = "normal";
+const char CN_QUORUM[]  = "quorum";
+const char CN_STATIC[]  = "static";
+const char CN_UNKNOWN[] = "unknown";
+
+}
+
+std::string Clustrix::to_string(Clustrix::Status status)
+{
+    switch (status)
+    {
+    case Status::QUORUM:
+        return CN_QUORUM;
+
+    case Status::STATIC:
+        return CN_STATIC;
+
+    case Status::UNKNOWN:
+        return CN_UNKNOWN;
+    }
+
+    mxb_assert(!true);
+    return CN_UNKNOWN;
+}
+
+Clustrix::Status Clustrix::status_from_string(const std::string& status)
+{
+    if (status == CN_QUORUM)
+    {
+        return Status::QUORUM;
+    }
+    else if (status == CN_STATIC)
+    {
+        return Status::STATIC;
+    }
+    else
+    {
+        MXB_WARNING("'%s' is an unknown status for a Clustrix node.", status.c_str());
+        return Status::UNKNOWN;
+    }
+}
+
+std::string Clustrix::to_string(Clustrix::SubState substate)
+{
+    switch (substate)
+    {
+    case SubState::NORMAL:
+        return CN_NORMAL;
+
+    case SubState::UNKNOWN:
+        return CN_UNKNOWN;
+    }
+
+    mxb_assert(!true);
+    return CN_UNKNOWN;
+}
+
+Clustrix::SubState Clustrix::substate_from_string(const std::string& substate)
+{
+    if (substate == CN_NORMAL)
+    {
+        return SubState::NORMAL;
+    }
+    else
+    {
+        MXB_WARNING("'%s' is an unknown sub-state for a Clustrix node.", substate.c_str());
+        return SubState::UNKNOWN;
+    }
+}