Add data_monitor meta-class to monitor accessing on domview_ui. - The monitoring is to prevent from reentry triggered by DOM events. - call 'do*' methods of a monitored class would try to lock domview_ui at first. - The method would not be executed actually if being fault to lock. - You can make monitor to print debug message with environment variable - DATA_MONITOR_DBG=1, or - DATA_MONITOR_DBG=2