Remove Stanza once callback called to avoid memory ballonning

Deleting session._expected[ID] when the callback will be processed
avoid this dict to grow over the time.

Author: CyrilPeponnet

xmpp/dispatcher.py

@@ -289,6 +289,7 @@ def dispatch(self,stanza,session=None,direct=0):
             user=0
             if type(session._expected[ID])==type(()):
                 cb,args=session._expected[ID]
+                del session._expected[ID]
                 session.DEBUG("Expected stanza arrived. Callback %s(%s) found!"%(cb,args),'ok')
                 try: cb(session,stanza,**args)
                 except Exception, typ: