Handle graceful shutdown on Heroku

[pietroalbini]

While the crates.io codebase gracefully shuts down when it receives a SIGINT or a SIGTERM, that doesn't work when running the application in production on Heroku. Now that DownloadsCounter landed in #3413 handling graceful shutdowns is actually needed, otherwise we'll lost up to a minute of download counts when restarting a dyno.

This seems to be caused by Heroku sending SIGTERM to all processes 30 seconds before killing the dyno and nginx forcefully shutting down after it receives the signal.