Improve message-type sharing

[udidahan]

Instead of copying the message classes between the various services, something similar can be achieved without creating a shared message assembly - use Linked Files to have a single copy of the message classes.

If this path is chosen, the Readme should also be updated.

[dvdstelt]

We considered that, but the namespaces were different in every single project. That's why we dismissed it as a solution.
However later on we discovered it would be impossible for RabbitMQ to work with different namespaces (since subscriptions would include namespace and we'd need to hack RabbitMQ transport to fix this). So we moved all messages into the same namespace. Which probably makes the linked files possible again.

Something worth considering.