inaccurate handling or weak references

[ods]

Describe the bug

Observed error:

  File "asyncio/events.py", line 81, in _run
    self._context.run(self._callback, *self._args)
  File "aiokafka/conn.py", line 387, in _idle_check
    idle_for = time.monotonic() - self._last_action
AttributeError: 'NoneType' object has no attribute '_last_action'

I've got this error almost simultaneously in multiple services connected to the same kafka instance. The most probable reason is a network failure.

There are other places where weak reference is used without check:

• https://github.com/aio-libs/aiokafka/blob/master/aiokafka/conn.py#L377
• https://github.com/aio-libs/aiokafka/blob/master/aiokafka/conn.py#L380
• https://github.com/aio-libs/aiokafka/blob/master/aiokafka/conn.py#L385
• https://github.com/aio-libs/aiokafka/blob/master/aiokafka/conn.py#L508

Environment:

• aiokafka version: 0.7.0