MXS-1848 Rename monitorDestroy to monitor_deactivate

MonitorDestroy() (renamed to monitor_destroy()) will be used for
actually destroying the monitor instance, that is, execute
destroyInstance() on the loaded module instance and freeing the
the monitor instance.

TODO: monitor_deactivate() could do all the stuff which is currently
      done to the monitor in config_runtime(), instead of just
      turning off the flag.

server/core/internal/monitor.h

@@ -52,7 +52,17 @@ void monitor_free(MXS_MONITOR *);
 
 void monitorStart(MXS_MONITOR *, const MXS_CONFIG_PARAMETER*);
 void monitorStop(MXS_MONITOR *);
-void monitorDestroy(MXS_MONITOR* monitor);
+
+/**
+ * @brief Mark monitor as deactivated
+ *
+ * A deactivated monitor appears not to exist, as if it had been
+ * destroyed.
+ *
+ * @param monitor
+ */
+void monitor_deactivate(MXS_MONITOR* monitor);
+
 void monitorStopAll();
 void monitorStartAll();