Merge data protocol into control protocol?

[BenediktBurger]

Original place: pymeasure/pyleco#89

Maybe we should scap the idea of the (PUB-SUB based) data protocol, and implement it via the control protocol.
That what make the whole setup more simple.

Reasoning

The data protocol infrastructure is not as flexible, as messages can flow only in one direction (from server to server).
Due to the different architectures of the data and control protocol networks, you might want to subscribe to a publishing actor, which you cannot reach via the data protocol.

We can, however use the control protocol to subscribe and to publish messages. (managing the subscribers manually)

This reflects the experience from #2 (comment) that it is quite simple to use the control protocol only. PUB-SUB does not save on bandwidth anyway.