Adds SIGTERM listener to gracefully shutdown the server

[HarryEMartland]

Kubernetes and other process managers send a SIGTERM to applications to give them time to shut down. This change adds a listener to this event which shuts the server down gracefully.

[HarryEMartland]

@dougwilson sorry I must have missed a notification adding the tags. When you say needs tests what kind of tests would you be looking for? Thanks

[UlisesGascon]

maybe @dougwilson can re-run the tests for Job #792.3. Seems like there was a error on node-sass building process (not relevant to the PR change) Direct Link

[HarryEMartland]

If someone could give me some pointers on how to test this I would greatly appreciate it. The issue is only really noticeable if a request takes a decent amount of time.

In sudo code I would want to test something like this;

Start app
Make a request to a slow endpoint
Send SIGTERM to process
Wait for the request to finish
Verify the request returned successfully
Verify the process exited

I have had a play with the existing tests and can see that there is code already sending SIGTERM. I am however unsure of how to add a new endpoint with a delay in it.