multi: send_txn returns Deferred.t instead of Ivar.t

Returning an Ivar.t gives the user the ability to fill it with a value.
Returning a Deferred.t on the other hand does not.

Author: seliopou

async/Async_OpenFlow.mli

@@ -175,7 +175,7 @@ module Chunk : sig
       :  t
       -> Client_id.t
       -> m
-      -> [ `Sent of Message.t Ivar.t | `Drop of exn ] Deferred.t
+      -> [ `Sent of Message.t Deferred.t | `Drop of exn ] Deferred.t
 
     val handshake : int -> (t, e, h) Stage.t
   end

async/Async_OpenFlow0x01.ml

@@ -103,8 +103,8 @@ module Controller = struct
     try begin
       send_txn t sw_id msg
       >>= function
-        | `Sent ivar -> Ivar.read ivar >>| f
-        | `Drop exn  -> return (Result.Error exn)
+        | `Sent def -> def >>| f
+        | `Drop exn -> return (Result.Error exn)
     end with Not_found -> return (Result.Error Not_found)
 
   let send_ignore_errors t sw_id msg =

async/Async_OpenFlow0x04.ml

@@ -138,8 +138,8 @@ module Controller = struct
     try begin
       send_txn t sw_id msg
       >>= function
-        | `Sent ivar -> Ivar.read ivar >>| f
-        | `Drop exn  -> return (Result.Error exn)
+        | `Sent def -> def >>| f
+        | `Drop exn -> return (Result.Error exn)
     end with Not_found -> return (Result.Error Not_found)
 
   let send_ignore_errors t sw_id msg =

async/Async_OpenFlowChunk.ml

@@ -257,7 +257,7 @@ module Controller = struct
     let ivar = Conn.add_txn conn m in
     send t c_id m
     >>| function
-      | `Sent _   -> `Sent ivar
+      | `Sent _   -> `Sent (Ivar.read ivar)
       | `Drop exn ->
         Conn.remove_txn conn m;
         `Drop exn