client lwt support for OCaml TLS

I still have some questions here:
- why is ``type client`` a polyvar of tuples, and ``type server`` a polyvar of polyvar-tuples (esp: ``int`` vs ```Port of int``)?
- the authenticator (which should verify the server certificate) is currently none -- a user of conduit needs to provide somehow a set of trusted anchors -- more generally: how to integrate authentication into conduit? there should be a unified concept to request authentication on the server side (``/FOO`` should be protected by either a valid user name or a valid client certifcate)
- should there really be a ```OpenSSL`` and ```TLS`` in the polyvar? or just one of them (``endp_to_client`` prefers ```TLS`` over ```OpenSSL`` (if ``tls.lwt`` is available))
- same for server side: where/when to specify which TLS/SSL stack to use? what about client authentication?