Add /pks/hashquery

  Add /pks/hashquery - an implementation of the SKS hash retrieval
  portion of the gossip protocol.

  hashquery takes a marshalled array of SKS hashes to retrieve and
  returns a marshalled array of the keys requested.

  (The marshalling functions essentially take the hash/key structures
   and flatten them to a byte stream with a preceding network order
   32 bit size value.)