Add a server-side Luxi implementation

This is a trivial code change, but it allows us to finally test the
send-receive code on both client and server sides via a simple
in-process server.

The unittest works, but it won't handle timeouts very nicely; it will
wait until the actual Luxi timeout expires, instead of using much
shorter timeouts as we could in the same process.

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