code: public CompletableFuture<Void> expire() {
      CompletableFuture<Void> future = new CompletableFuture<>();
      context.executor().execute(() -> {
        if (keepAlive != null)
          keepAlive.cancel();
        state.setState(Session.State.EXPIRED);
        future.complete(null);
      });
      return future;
    }
docs: Expires the manager.
  
  @return A completable future to be completed once the session has been expired.