This can be useful if:
1. You can't rely on a persistent file system (e.g. Heroku)
2. Your application is open source (e.g. you can't commit the key)
By keeping a consistent value in the environment variable, your users will
have consistent sessions without relying on the file system.
Usage:
makeSessionBackend _ = fmap Just $ envClientSessionBackend 120 "SESSION_KEY"