Enable a `clean` target.
Refactoring of dht_net, first round.

Clean up comments. Read through code adding @todo entries where something
has to be done. Also clean up the send_query code path.
Refactor the `dht_net` module.

* Use more canonical idioms for passing around data.
* Ready `dht_bt_proto` for some EQC testing.
* Clean up code which were not really consistent.