message: next_if_allowed is no more
diff: @@ -48,7 +48,7 @@ pub struct Scheduler<T, R> {
  queue: DelayQueue<T>,
  /// Metadata for all currently scheduled messages. Used to detect duplicate messages.
  scheduled: HashMap<T, ScheduledEntry>,
  - /// Messages that are scheduled to have happened, but have been held using `next_if_allowed`.
  + /// Messages that are scheduled to have happened, but have been held using `hold_unless`.
  pending: HashSet<T>,
  /// Incoming queue of scheduling requests.
  #[pin]
  