Consumer Shutdown Testing: Shard Reallocation

[markglh]

When processing from multiple shards (1 consumer), we need to assert the following.

• Reducing the shard count from 2 to 1 (in Kinesis) should result in the consumer continuing to consume from the remaining shard. Kinesis will reallocate messages to the remaining shard automatically. We need to ensure the consumer continues seamlessly.
• It's possible that the ConsumerManager within the client will shutdown the whole consumer when a shard is removed as it calls shutdown on the KCLWorker, which isn't shard specific.
• Note also that there are two shutdown scenarios in the manager, one when shutting down a shard gracefully and one due to an exception - this is testing the former.