Simplify failed master detection

No longer depends on monitor events as the other operations do not
either. The active_event/new_event detection was removed, as these
only protect against a rare situation. A similar feature which
protects all the cluster modifications will be implemented later.

include/maxscale/monitor.h

@@ -221,7 +221,6 @@ typedef struct monitored_server
     int mon_err_count;
     uint64_t mon_prev_status;       /**< Status before starting the current monitor loop */
     uint64_t pending_status;        /**< Status during current monitor loop */
-    bool new_event;                 /**< Whether an action was taken on the last event */
     int64_t disk_space_checked;     /**< When was the disk space checked the last time */
     struct monitored_server *next;  /**< The next server in the list */
 } MXS_MONITORED_SERVER;