-track contact failures, last replied, and last requested. use this to provide a 'contact_is_good' property on Contact objects
-ensure no duplicate contact objects are created
-remove confusing conflation of node id strings with Contact objects, update docstrings
-move RPC failure tracking to a callback/errback pair in sendRPC (so the contact is only updated once)
-handle seed nodes during the join sequence by setting their node ids after they initially reply to our ping
-name all of the kademlia RPC keyword args, remove confusing **kwargs and dictionary parsing
-add host ip/port to DHT send/receive logging to make the results comprehensible when running many nodes at once
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