Do not enable Event.next throttling when Event.next is not disabled

It slows down all synchronous API calls that create tasks, like VM.start.

ocaml/xapi/xapi_event.ml

@@ -476,7 +476,7 @@ let unregister ~__context ~classes =
 
 (** Blocking call which returns the next set of events relevant to this session. *)
 let rec next ~__context =
-  let batching = !Xapi_globs.event_next_delay in
+  let batching = if !Constants.use_event_next then !Xapi_globs.event_from_task_delay else !Xapi_globs.event_next_delay in
   let session = Context.get_session_id __context in
   let open Next in
   assert_subscribed session ;