Automatically delete expired sessions from database

[airblade]

It's not clear what a good time to do this is.

We don't want to do it during a user's request. But we don't want to assume a background job system.

Perhaps the answer is a scope on QuoVadis::Session and a snippet in the README showing how to use it from, say, a cron job.

[airblade]

Quo Vadis is built for Rails so we can assume a background job system.