Obtain the new descriptors and network status documents by signals instead of every 5min

The network status and descriptors are obtained every 5min https://gitweb.torproject.org/sbws.git/tree/sbws/lib/relaylist.py#n144. Is there any reason for that?. I think torflow obtain them as they come. It could be done with stem using:

controller.add_event_listener(
            listener_fn, EventType.NS, EventType.NEWDESC)

Also, instead overwriting those values, Relay could have a dictionary (key would be the timestamp) with a subset of the attributes of document, in case we might need to calculate median/mean of those attributes

Edit: fix typo