Cohttp_eio: Relax requirement on Server.connection_handler.

	Server.connection_handler does not rely on its argument being
        a socket, a two way flow is sufficient. Relaxing the type
        constraint enables eg. unit testing with in-memory buffer.

cohttp-eio/src/cohttp_eio.mli

@@ -107,7 +107,7 @@ module Server : sig
   val connection_handler :
     handler ->
     #Eio.Time.clock ->
-    #Eio.Net.stream_socket ->
+    #Eio.Flow.two_way ->
     Eio.Net.Sockaddr.stream ->
     unit
   (** [connection_handler request_handler] is a connection handler, suitable for