MXS-2271 Monitor modifications always go through Monitor::configure()

Previously, runtime monitor modifications could directly alter monitor fields,
which could leave the text-form parameters and reality out-of-sync. Also,
the configure-function was not called for the entire monitor-object, only the
module-implementation.

Now, all modifications go through the overridden configure-function, which calls the
base-class function. As most configuration changes are given in text-form, this
removes the need for specific setters. The only exceptions are the server add/remove
operations, which must modify the text-form serverlist.

maxutils/maxbase/include/maxbase/worker.hh

@@ -423,7 +423,7 @@ public:
      *
      * @return True if the thread could be started, false otherwise.
      */
-    bool start();
+    virtual bool start();
 
     /**
      * Waits for the worker to finish.