title: Kafka python Consumer group session timed out
body: <p>I use confluent-kafka v1.3.0 and I have following problem with consumer group session timeout. 
  My config looks like:</p>
  
  <pre><code>c['KAFKA'] = {
      'bootstrap.servers': 'host.docker.internal:9104',
      'consumer': {
          'group.id': 'consumer',
          'enable.auto.commit': True,
          'default.topic.config': {
              'auto.offset.reset': 'earliest
          },
          'heartbeat.interval.ms': 100000,
          'max.poll.interval.ms': 300000,
          'session.timeout.ms': 100000
      },
  }
  </code></pre>
  
  <p>and logic in code like:</p>
  
  <pre><code>consumer.subscribe('database_changes')
  
  with ThreadPoolExecutor(max_workers=500) as executor:
      while True:
          msg = consumer.poll(100)
          if msg is not None:
              executor.submit(process_message, msg)
  </code></pre>
  
  <p>Code in function process message waits a few ms because it's really simple logic. Everything works good but every moment I got this error:</p>
  
  <pre><code>{"asctime":"2020-04-27 08:42:25,759","levelname":"WARNING","name":"services.kafka","message":"SESSTMOUT [rdkafka#consumer-2] [thrd:main]: Consumer group session timed out (in join-state started) after 30131 ms without a successful response from the group coordinator (broker 0, last error was Success): revoking assignment and rejoining group"}
  </code></pre>
  
  <p>these rebalacing greatly hampers the whole process. </p>
  
  <p>Does anyone have an idea of what can be wrongly set? I suspect a not working heartbeat, but I don't know how it verify or better fix.</p>
  
  <p>Thanks</p>