Uniformly expose connection types

[seliopou]

The IO module type includes a conn type which—if I understand it correctly—should be a type that includes information about the client's connection to the server. Right now, the async implementation simply sets this to unit, and in fact conn doesn't seem to be used by that implementation at all. But while the lwt (unix) implementation does use it, it also tacks on an additional Cohttp.Connection.t value with it as well.

The IO.conn type's meaning and usage should be consistently recognized and applied by all backends. Related to inhabitedtype/ocaml-webmachine#19.