Fix that released all fetch waiters introduced error messages for idle Faust worker

[forsberg]

Checklist

• I have included information about relevant versions
• I have verified that the issue persists when using the master branch of Faust.

Steps to reproduce

Run a faust worker that waits for messages on a topic with no or very little traffic.

Expected behavior

Worker should sit and do nothing.

Actual behavior

I get ERROR level messages as follows:

[2020-11-17 16:19:03,695] [708969] [ERROR] Exception in callback Fetcher._create_fetch_waiter.<locals>.<lambda>(<Future cancelled>) at /home/forsberg/.virtualenvs/ferrostream/lib/python3.8/site-packages/aiokafka/consumer/fetcher.py:446
handle: <Handle Fetcher._create_fetch_waiter.<locals>.<lambda>(<Future cancelled>) at /home/forsberg/.virtualenvs/ferrostream/lib/python3.8/site-packages/aiokafka/consumer/fetcher.py:446> 
Traceback (most recent call last):
  File "/usr/lib/python3.8/asyncio/events.py", line 81, in _run
    self._context.run(self._callback, *self._args)
  File "/home/forsberg/.virtualenvs/ferrostream/lib/python3.8/site-packages/aiokafka/consumer/fetcher.py", line 446, in <lambda>
    lambda f, waiters=self._fetch_waiters: waiters.remove(f))
KeyError: <Future cancelled>

Problem goes away if I revert 361b09d (releasing all fetch waiters).

Problem also seems to appear during rebalance for busy workers, i.e workers where the input topic is busy.

Versions

• Python version 3.8.5
• Faust version faust-streaming 18230a7
• Operating system Linux
• Kafka version 2.5.0
• RocksDB version (if applicable) N/A

[patkivikram]

This is fixed in release 0.3.0 with PR #45