overlap the onDocumentStart in multixmlstream to store the root element
overlap the onElement in multixmlstream to add the elements into the
root element as a child

refactory the construction of the x2Server
1. in __init__, add observer to moditor the related event:
	a. ping request
	b. x2 iri event
2. when trigger the function, add onetime to process next action in
STREAM_END_EVENT
	a. send out related response
	b. record it into log
	
add the test_irievent to verify the procedure after recv the iri-event