Fix node finding algorithm.
Fix node finding algorithm.

When calling find(query_id) on a table with id 'my_id', it used to look
only in the bucket numbered by 'query_id ^ my_id', and sort the results
by comparing 'my_id ^ id1' with 'my_id ^ id2'.

My understanding of the Kademlia algorithm is that all buckets should
be searched, and they should be sorted by comparing 'query_id ^ id1' with
'query_id ^ id2'.

In this commit, I also replace 'sort_by' by 'sort_by_key', so the number
of XOR computations goes from Θ(n log(n)) to Θ(n).

divius/rust-dht,divius/rust-dht