Add a real async publishing pipeline to MemBus

[flq]

This issue takes over from #16 and plans out the necessary steps (more or less) to get real async publishing into MemBus.

Aim

• Ability to await the handling of a published message
• Ability to implement subscriptions that can be truly async.

TODO

• [ ] Introduce an IAsyncSubscription interface that returns Task instead of void.
• [ ] IDisposableAsyncSubscription- this one needs some thinking with regard to cancellation token
• [ ] There are a number of ISubscription implementations which would need async counterparts, last but not least FilteredSubscription
• [ ] CompositeSubscription - what makes sense here? If you have several async subscriptions, would you want to await all? Or none? Make this configurable?
• [ ] IAsyncSubscriptionShaper for IAsyncSubscription
• [ ] Configuration Entry points for the new pieces.

Then there are the things around Flexible subscriptions. New candidates would now be

• [ ] Non-void methods
• [ ] Methods that return a message -> Task<M> should be possible

Progress will be merged into the branch async-pipeline until it's ready.