I'm doing this change for future performance optimisations. Currently
we use the Luxi interface just as a client, so not in the hot path,
but when we'll use this as a server interface, we're interested to
both reduce the space and time consumption of the interface.

We have to simultaneous changes here:

- switch from using socket-related function (sendto, recv, etc.) to
  handle-based functions, since the standard network library doesn't
  work with sockets
- switch from using Strings for the internal buffer to strict
  ByteStrings; the only downside is that we now have the issue of
  decoding/encoding from binary to UTF-8 strings, a fact which brings
  its own issues into the mix (we have to check for failed decodings,
  etc.); but this is similar to what we'll have to handle on the
  Python side when moving to Python 3.x

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>