Run NotificationHandlers in parallel

[Neo-vortex]

One of the philosophy of notification is to notify different handlers at the same time (hopefully). currently what we do is :

 for (int i = 0; i < handlers.Length; i++)
                {
                    try
                    {
                        await handlers[i].Handle(notification, cancellationToken).ConfigureAwait(false);
                    }
                    catch (global::System.Exception ex)
                    {
                        exceptions ??= new global::System.Collections.Generic.List<global::System.Exception>();
                        exceptions.Add(ex);
                    }
                }

MediatR 12 has implemented an option to define if Notifications handlers should run parallel or not. here is a link to a video demonstrate it : https://www.youtube.com/watch?v=hNxVjNO6RX4 Mediator library should add an option for these execution strategies.

imagine these two notification handlers :

public sealed class GenericNotificationHandler<TNotification> : INotificationHandler<TNotification>
    where TNotification : INotification
{
    public async ValueTask Handle(TNotification notification, CancellationToken cancellationToken)
    {
               // very long operation
                await Task.Delay(10000, cancellationToken);
    }
}

public sealed class GenericNotificationHandler2<TNotification> : INotificationHandler<TNotification>
    where TNotification : INotification
{
    public async ValueTask Handle(TNotification notification, CancellationToken cancellationToken)
    {
        await Task.Delay(2000, cancellationToken);
    }
}

with current implementation the handling time for all the handlers would be about ~ 12000 ms but with the new implementation it would be about ~ 10000 ms