[Fearture] Use asyncio internally to reduce number of threads

[jan-janssen]

The ZMQ communication between processes is the natural target for asyncio:

administrative thread                 worker process
       │                                    │
       │ ZMQ send(task)                     │
       ├───────────────────────────────────►│
       │                                    │ calculate()
       │                                    │
       │        thread waits                │
       │        thread waits                │
       │        thread waits                │
       │                                    │
       │◄───────────────────────────────────┤
       │ ZMQ recv(result)                   │