MXS-2384 Update servers concurrently in MariaDB-Monitor

All servers are now updated in their own threads simultaneously. This
should reduce the possibility of having significantly different gtid:s
shown for different servers.

server/modules/monitor/mariadbmon/mariadbserver.hh

@@ -116,7 +116,6 @@ public:
         bool log_slave_updates = false; /* Does the slave write replicated events to binlog? */
     };
 
-
     /* Monitored server base class/struct. MariaDBServer does not own the struct, it is not freed
      * (or connection closed) when a MariaDBServer is destroyed. */
     mxs::MonitorServer* m_server_base = NULL;
@@ -168,6 +167,9 @@ public:
      */
     std::string diagnostics() const;
 
+    void update_server(bool time_to_update_disk_space,
+                       const mxs::MonitorServer::ConnectionSettings& conn_settings);
+
     /**
      * Query this server.
      */