Start the SNMP's protocol handler and the updater thread
		user_func is a reference to an update function, ran every 'refresh' seconds.

def start(self, user_func, refresh):
		"""
		Start the SNMP's protocol handler and the updater thread
		user_func is a reference to an update function, ran every 'refresh' seconds.
		"""
		self.update=user_func
		self.refresh=refresh
		self.error=None

		# First load
		self.update()
		self.commit()

		# Start updater thread
		up = threading.Thread(None,self.main_update,"Updater")
		up.daemon = True
		up.start()

		# Main loop
		while up.isAlive(): # Do not serve data if the Updater thread has died
			try:
				self.main_passpersist()
			except:
				up._Thread__stop()
				raise