First implementation of minimal Broker API

1. Message content is transparent to the broker, so inspection is not possible. Whether this is really needed or not, I can only tell from the current use-case, where it's not. If there are ambiguities with the message type, then the type is too generic in my opinion.
2. The application can implement rerouting if needed. The broker just informs on receive of a certain message, but offloading the routing to the broker I don't see valuable right now.