[QUESTION] Restarting `AIOKafkaConsumer` after `AIOKafkaConsumer.stop()`

Hi,

I have a use case where I'm injecting a `AIOKafkaConsumer` instance into a class. The class has its own `start` and `stop` methods that, amongst other things, call the `start` and `stop` methods of the consumer class. The issue is that I'm failing the `assert self._fetcher is None` check in `AIOKafkaConsumer.start()` method.

Am I right in thinking this should work?

```py
import asyncio

from aiokafka import AIOKafkaConsumer

async def main() -> None:
consumer = AIOKafkaConsumer("test_topic", bootstrap_servers="localhost:9092")

await consumer.start()
await consumer.stop()
await consumer.start()

if __name__ == "__main__":
asyncio.run(main())
```

I suppose I could pass the class a function that creates the consumer instance, but it feels a bit less neat.
```py
import asyncio

from aiokafka import AIOKafkaConsumer

async def main() -> None:
def get_new_consumer() -> AIOKafkaConsumer:
return AIOKafkaConsumer("test_topic", bootstrap_servers="localhost:9092")

consumer = get_new_consumer()

await consumer.start()
await consumer.stop()

consumer = get_new_consumer()

await consumer.start()

if __name__ == "__main__":
asyncio.run(main())
```