activity: Make echo the first stage of the pipeline

According to the OpenFlow specifications, controllers should implement
the echo response at the lowest-level possible.

Author: seliopou

async/Async_OpenFlow.mli

@@ -151,7 +151,7 @@ module Chunk : sig
     val set_idle_wait : t -> Time.Span.t -> unit
     val set_kill_wait : t -> Time.Span.t -> unit
 
-    val echo : (t, h, h) Stage.t
+    val echo : (t, e, e) Stage.t
     val handshake : int -> (t, e, h) Stage.t
   end
 

async/Async_OpenFlow0x01.ml

@@ -157,7 +157,7 @@ module Controller = struct
     let open ChunkController in
     let stages =
       (local (fun t -> t.sub)
-        (handshake 0x01 >=> echo))
+        (echo >=> handshake 0x01))
       >=> openflow0x01 in
     run stages t (listen t.sub)
 end

async/Async_OpenFlow0x04.ml

@@ -71,7 +71,7 @@ module Controller = struct
     let open ChunkController in
     let stages =
       (local (fun t -> t.sub)
-        (handshake 0x04 >=> echo))
+        (echo >=> handshake 0x04))
       >=> openflow0x04 in
     run stages t (listen t.sub)