Feature Request: Graceful Shutdown

[ColinSullivan1]

Allow for a graceful shutdown minimizing message loss.

Some suggestions:

• Implement a signal handler to shutdown gracefully. This may be important in k8s. See nats-surveyor.
• On the NATS side use Conn.Drain() when exiting.
• Investigate the Kafka Consumer side - check if there is a way to unsubscribe and process what is buffered.

[variadico]

The following are done, closing.

• [x] Implement a signal handler to shutdown gracefully
• [x] Add Conn.Drain()
• [x] The underlying library already handles producer graceful shutdown