Fire cron event if auto-purge value is updated

[frankiejarrett]

Currently the cron event that auto-purges records based on their age is set to run once daily.

However, if the value in this setting is set to 90 days and the user changes it to 1 day they would likely expect that the next time they visit the stream that all records older than 1 day would be gone.

So instead of potentially waiting up to 24 hours for the cron to fire, we should think about forcing it to run each time this value is changed.