Drain connections before shutdown

[jutley]

When using kafka-proxy as a Kubernetes pod sidecar container, the main application may need to send data to Kafka during its graceful shutdown period. However, kafka-proxy closes very quickly, terminating any live connections that are open.

It would be great to be able to configure kafka-proxy so that it doesn't terminate until all existing connections are closed. This would probably need some configurable timeout as well.