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/CMakeLists.txt

@@ -1,4 +1,5 @@
 add_library(clustrixmon SHARED
+  clustrix.cc
   clustrixmon.cc
   clustrixmonitor.cc
   )