Pass HTTP request to ctx function in Graphql_lwt.Server.start

[andreas]

This PR changes the type of Graphql_lwt.Server.start:

(* Before this PR *)
val start : ?port:int -> ctx:(unit -> 'ctx) -> 'ctx Schema.schema -> unit Lwt.t

(* After this PR *)
val start : ?port:int -> ctx:(Cohttp.Request.t -> 'ctx) -> 'ctx Schema.schema -> unit Lwt.t

This allows library users to construct a context that depends on the incoming HTTP request, e.g. cookies, headers, etc. Related to https://github.com/andreas/ocaml-graphql-server/issues/43#issuecomment-378058396.