Implement key fetching for Key Manager.

Pseudocode:

#
# for now, maybe just pick a particular nickserver, instead of trying many.
# We can gradually add more complexity and more security to this point, which is the most important one
#
function fetch_key_from_nickserver(key)
  randomly pick a subset of the available nickservers we know about
  send a tcp request to each in this subset in parallel
  first one that opens a successful socket is used, all the others are terminated immediately
  make http request
  parse json for the keys
  return keys
end

Ref: https://pad.riseup.net/p/oWbxG2orbbnK

(from redmine: created on 2013-04-14, closed on 2013-04-22)