Allow openssl conduit-lwt-unix users to control their client Ssl.contexts

[lippirk]

I am making this patch because I'd like to be able to control the
lifetime of my openssl client context directly.

Motivation for this includes the fact that the global
Conduit_lwt_unix_ssl.Client.default_ctx is not sufficient in the case
that you want to update an ssl context's verify location (it is
necessary to use a new ssl context).

Example usage with cohttp:

(* three types of context :D *)
let client_ssl_context : Ssl.context = Conduit_lwt_unix_ssl.Client.create_ctx () in
(* some ssl setup *)
let* (ctx : Conduit_lwt_unix.ctx) = Conduit_lwt_unix.init ~client_ssl_context () in
let ctx : Cohttp_lwt_unix.Client.ctx = Cohttp_lwt_unix.Client.custom_ctx ~ctx () in
let* _ = Cohttp_lwt_unix.Client.call ~ctx `POST ~body:"blah" uri in
...

[lippirk]

closing in favour of #390