Add "404 lifetime" to module to help with performance tuning

Here's my patch on some initial work.

Cache clear always deletes
Cron checks against state of when 404 was last generated
Standardized some calls to strings and such to keep code DRY

@jnicola, The patch no longer applies after the merge of the meta tag change. Can you please move this change into a MR. It will be easier for review and any feedback that might be needed.

I'll try and get that out here soon!

Hi @jnicola,

Are you using a Cron manager?
Personally, I use Ultimate Cron → , which allows me to work around the issue you mentioned by setting a broader schedule for the fast_404_generator cron (e.g., once an hour or even once a day), or even by disabling the cron on certain environments so that the HTML file is only regenerated when the cache is cleared.

I'm familiar with it.

Unfortunately, you can't always utilize it. Whether it's the hosting services doesn't recommend it, or you've got uninformed obstinate devops team members who make you want to pull your hair out even though you shave your head...

For those situations, a minimum lifetime is useful :)