Fix shutdown and race-condition in consumer-group example

[skidder]

The Kafka consumer groups example has two issues:

1. A race condition causes the ready channel to be reassigned before the consumer starts, leading to the main go-routine reading from a different ready channel than the one that is closed in the consumer Setup function.
2. The consumer go-routine doesn't exit cleanly when a termination signal is sent. But the termination signal handling wasn't configured because of the aforementioned race condition, so this wouldn't have worked regardless.

This PR fixes both problems by reassigning the ready channel variable only after a rebalance, and by using a cancellable context and waitgroup to ensure that the Kafka client is closed cleanly upon receiving a termination signal.

[bai]

Many thanks!