Decouple api from zmq backend

[mraspaud]

At the moment, the zmq calls are tightly woven into the postroll code, making it difficult to add new backends for postroll.

Abstracting the current code and isolating zmq calls behind a simple backend api would allow us to add new backends. New backends could be interesting for several reasons, among which at least a couple which I think will help us develop cleaner postroll based application:

• a pytest or testing backend, which would not actually make any network calls but instead would allow devs to know what has sent and recieved without the need to patch or mock,
• an asyncio backend for applications that just need simple concurrency without threads.

Other backends are of course possible, like rabbitmq or other more specific libraries (for kubernetes maybe?)