asyncio support?

Hi there,

due to issues like #148 and #138 I was wondering if it was possible to implement asyncio support in order to simultaneously process new mqtt events and do other stuff concurrently.
asyncio is suggested by adafruit as the way to deal with [concurrent tasks](https://learn.adafruit.com/cooperative-multitasking-in-circuitpython-with-asyncio/concurrent-tasks). So I was wondering, why the library does not support asyncio (yet?).

Best regards,
Rico

Start by reviewing issues #148 and #138, then read the linked Adafruit CircuitPython asyncio guidance. Define which MiniMQTT APIs and event-processing behavior must change so MQTT events can run alongside other asyncio tasks; done means the intended concurrent usage is supported and documented by appropriate tests.

Written by the indexing model from the issue text.