Implement key refreshing in Key Manager

Pseudocode:

#
# update the user's db of validated keys to see if there are changes.
#
function refresh_keys()
  for each key in the soledad database (that should be checked?):
      newkey = fetch_key_from_nickserver()
      if key is about to expire and newkey complies with the renewal paramters:
          replace key with newkey
      else if fingerprint(key) != fingerprint(newkey):
          freak out, something wrong is happening? :)
          may be handle revokation, or try to get some voting for a given key and save that one (retrieve it through tor/vpn/etc and see what's the most found key or something like that.
      else:
          everything's cool for this key, continue
      end
  end
end

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

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