Swap `pika` for `aio-pika`

[ric-evans]

Currently while the rabbitmq backend technically has async functions, using them does nothing as the rabbitmq parts are synchronous. Switch to aio-pika to get the full async experience:

https://aio-pika.readthedocs.io/en/latest/quick-start.html

Note that this would help with #58, as you could then run multiple async sends in parallel.

[ric-evans]

This may solve the problem with heartbeats during long-running processing. TODO: check if aio-pika has a background heartbeat