Aync first Event Aggregator

[nigel-sampson]

As part of 4.0 we're moving to async first across all major interfaces. This issue summarizes the work required for the EventAggregator.

I want to approach this in few steps all of which are breaking changes.

1. Move from three interfaces IHandle<T>, IHandleWithCoroutine<T> and IHandleWithTask<T> to a single interface IHandle<T> that supports Task based subscription:

Task HandleAsync(TMessage message, CancellationToken cancellationToken);

2. Publish PublishAsync will return a Task that completes when all handlers have finished and will take a cancellation token.
3. Handlers will be called in parallel. This will be extensible.
4. The Publish overloads of Current Thread, Background Thread and UI Thread will be simplified (we won't need the various overloads of UI thread).
5. Subscribe will be changed to support similar semantics SubscribeOnBackgroundThread etc.

This rolls up issues covered by #409 and #416.