Undo deprecation of connection value

In #798 the connection value has been deprecated with the claim it is
useless.  However, in fact it is used at least with the Mirage backend in
order to track the connections for logging etc.  (See last two comments of
the PR.) In over 3 years no replacement has been added, so this change
removes the deprecation until some alternative exists.  (Which could be
possibly Mirage only.)

Author: ansiwen

cohttp-eio/src/server.ml

@@ -2,7 +2,7 @@ open Utils
 module IO = Io.IO
 
 type body = Body.t
-type conn = IO.conn * Cohttp.Connection.t [@@warning "-3"]
+type conn = IO.conn * Cohttp.Connection.t
 type writer = Http.Request.t * IO.oc
 type response = writer -> unit
 

cohttp-lwt/src/server.ml

@@ -1,6 +1,6 @@
 open Lwt.Infix
 module Header = Cohttp.Header
-module Connection = Cohttp.Connection [@@warning "-3"]
+module Connection = Cohttp.Connection
 
 module Make (IO : S.IO) = struct
   module IO = IO

cohttp-mirage/src/static.ml

@@ -18,7 +18,7 @@
  *)
 
 module Key = Mirage_kv.Key
-module Connection = Cohttp.Connection [@@warning "-3"]
+module Connection = Cohttp.Connection
 
 module HTTP (FS : Mirage_kv.RO) (S : Cohttp_lwt.S.Server) = struct
   open Lwt.Infix

cohttp/src/cohttp.ml

@@ -2,7 +2,7 @@ module Accept = Accept
 module Auth = Auth
 module Body = Body
 module Conf = Conf
-module Connection = Connection [@@deprecated "Connection.t values are useless."]
+module Connection = Connection
 module Code = Code
 module Cookie = Cookie
 module Header = Header

cohttp/src/server.ml

@@ -2,7 +2,7 @@ module type S = sig
   module IO : S.IO
 
   type body
-  type conn = IO.conn * Connection.t [@@warning "-3"]
+  type conn = IO.conn * Connection.t
   type response
 
   type response_action =