CP-51692: Event.next: use same batching as Event.from

Event.next is deprecated, but was allowed to use a lot more CPU in a tight loop
than Event.from.

No feature flag for this one, because Event.next is deprecated.

Signed-off-by: Edwin Török <edwin.torok@cloud.com>

ocaml/xapi/xapi_event.ml

@@ -470,6 +470,10 @@ let unregister ~__context ~classes =
 
 (** Blocking call which returns the next set of events relevant to this session. *)
 let rec next ~__context =
+  let batching =
+    Throttle.Batching.make ~delay_before:Mtime.Span.zero
+      ~delay_between:Mtime.Span.(50 * ms)
+  in
   let session = Context.get_session_id __context in
   let open Next in
   assert_subscribed session ;
@@ -489,11 +493,12 @@ let rec next ~__context =
     )
   in
   (* Like grab_range () only guarantees to return a non-empty range by blocking if necessary *)
-  let rec grab_nonempty_range () =
+  let grab_nonempty_range =
+    Throttle.Batching.with_recursive_loop batching @@ fun self () ->
     let last_id, end_id = grab_range () in
     if last_id = end_id then
       let (_ : int64) = wait subscription end_id in
-      grab_nonempty_range ()
+      (self [@tailcall]) ()
     else
       (last_id, end_id)
   in