Axon Framework 5 support #335

With the introduction of Axon Framework 5, many things have changed. We need to support these new things in the plugin as well.

There are several things to think of here:

• Support of (non-annotated) entity models
• Support of DCB
• Injection of models
• New test fixtures
• Warning of @Internal annotations
• New asynchronous messaging and support of ProcessingContext
• Better UnitOfWork instrumentation

Development on this should start somewhere between the M2 and M3 release. It is currently unclear if the API needs to change a lot.