title: Issue in rewinding to oldest available message for KafkaConsumer
body: <p>I have rewinded fetch offset for one Kafka consumer in group G1 to off1 by following code:</p>
  
  <pre><code>  consumer1.set_topic_partitions((topic, partition, off1))
  </code></pre>
  
  <p>In the above line, off1 is offset of oldest available message for the topic in corresponding partition.</p>
  
  <p>Now I am instantiating a Kafka consumer in different group G2 like following:</p>
  
  <pre><code> consumer2 = KafkaConsumer(bootstrap_servers=broker,
                                    auto_offset_reset='smallest',
                                    auto_commit_enable=True,
                                    auto_commit_interval_ms=3000,
                                    group_id='G2'
                                    )
  </code></pre>
  
  <p>Here I am reading the fetch offset as off1 which is same as that I reset for the consumer1 in group G1. I think that should not happen as for different groups offset should be different. If anyone clarifies, I will be really grateful. Thanks in advance.</p>