Ensure a graceful/orderly shutdown so the server doesn't exit mid file write. Use: * [CancellationToken](https://docs.rs/tokio-util/0.7.17/tokio_util/sync/struct.CancellationToken.html) * [TaskTracker](https://docs.rs/tokio-util/0.7.17/tokio_util/task/task_tracker/struct.TaskTracker.html) * Example: https://tokio.rs/tokio/topics/shutdown
