listen for termination signals and gracefully shut down the app

[mrparkers]

the application needs to listen for termination signals such as SIGINT (which is typically sent when you use ctrl+C to stop the app locally) and SIGTERM (which is sent by Kubernetes when trying to shut down the running pod). when these signals are received, we need to stop listening for new slack messages, terminate any active connections to the database, then exit the process.

references:

• listening for termination signals: https://github.com/liatrio/blog-vault-mongodb-dynamic-credentials/blob/234000347bfc20c3d47063ccda77448ad25159d6/example_app/src/index.js#L42-L48
• stopping the process: https://github.com/liatrio/blog-vault-mongodb-dynamic-credentials/blob/234000347bfc20c3d47063ccda77448ad25159d6/example_app/src/index.js#L57-L68