add Connection.mli

lib/server/connection.mli

@@ -0,0 +1,15 @@
+open Irc
+
+type t
+
+val make :
+  router:Router.t ->
+  server_info:Server_info.t ->
+  addr:Unix.sockaddr ->
+  t
+
+val close : ?reason:string -> t -> unit
+
+val outbox : t -> Outbox.t
+val on_msg : t -> Msg.t -> unit
+val on_ping : t -> (unit, unit) result