Tls_lwt: Do not resolve hostnames anymore, optional argument ?fd

* The resolve function is deleted, connect functions now take an
  Lwt_unix.sockaddr instead on a host:port tuple
* There is a new ~peer argument to connect functions for TLS needs

* There is an optional ?fd argument so that the user can keep a
  reference to the fd used (and be able to do sockopts on it, for
  example).