Expose connection information in Rd.t

[seliopou]

Cohttp connection handlers take connection information as their second argument. This argument does not appear in handlers that webmachine generates because the type of this connection information is not uniformly specified in cohttp. There is a IO.conn type that cohttp exposes, but this is simply unit for the async implementation. This is different from the type of the connection argument in async connection handlers.

Ideally, the IO module type in this library would include a conn type that would specify the connection type, and that would be compatible with the lwt and async IO modules exported by cohttp.