Add subscriptions via control protocol

[BenediktBurger]

Idea:

• you can subscribe to a Component A calling its register_subscription method
• For each published message (via data protocol), this Component A will call add_subscription_message (TBD) on all components which registered themselves previously. "Data" is the content of the data message. Conversation_id is the same. If a component is not reachable, it is removed from the list of subscribers.
• subscription_message will call handle_subscription_message with an artificial Data message.

This allows to subscribe to events on other computers without creating a dedicated data protocol network.

[BenediktBurger]

Reasoning:
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 pymeasure/leco-protocol#2 (comment) that it is quite simple to use the control protocol only. PUB-SUB does not save on bandwidth anyway.

[BenediktBurger]

Implementation ideas:

• how to make sure, that you are still subscribed, even if the sender is restarted (what does ZMQ do with PUB-SUB: re subscribe?)
  • SUB socket definition: destroy queue, so no reconnect if the publisher is stopped.
• how to remove a non responding receiver from the list? when to check for replies to failed deliveries?
• use "notifications", so typically no answer? (does this work with our JSON-RPC server? I think it did not work for the original one)
  • see Add JSON-RPC notifications #127 for JSON notifications