Gracefully close long-lived connections to backing services when processes end

[gnarea]

Fastify helps with this with an onClose hook which the Mongoose plugin uses, but we're not currently calling fastify.close() following a SIGINT/SIGTERM. https://github.com/hemerajs/fastify-graceful-shutdown can do that for us, but it's a bit of an overkill and has some caveats (as mentioned in the README).

All other processes (i.e., the gRPC server and background queues) don't currently offer a way to release resources upon process termination, so we'll have to implement something custom.