[Super Cache] Ensure cron schedule is setup whenever the config changes

[thingalon]

Possibly related: https://wordpress.org/support/topic/empty-cache-directories/

When you set a preload schedule in wp-super-cache, the cron job was only being set up if one was already scheduled. If one wasn't for any reason, then it did not add one.

This PR changes the way that cronjobs are managed: Any time the settings are edited, it ensures that there is a cron if one should be there, and there isn't one if not. If the interval gets edited, then the cronjob gets destroyed (if it exists) and recreated.

Note: this will not schedule a cronjob if preload mode is not enabled. I wanted to verify with @donnchawp that's the correct / expected behaviour.

Proposed changes:

• Ensure that preload crons are scheduled (or cancelled) on settings edit

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

n/a

Does this pull request change what data or activity we track or use?

no

Testing instructions:

• Try turning on preload mode, ensure a cron is appropriately scheduled
• Turn off preload mode, make sure there is no cron
• Turn on preload mode and edit the interval, ensure the correct interval gets set.

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Super Cache plugin:

• Next scheduled release: February 7, 2023.
• Scheduled code freeze: January 30, 2023.

[dilirity]

Thank you for this! I remember a while back I was testing preload for a support ticket and I couldn't re-schedule a preload after deleting it with WP Crontrol or something.

This update makes things work as expected.

[thingalon]

Cherry-picked into 1.9.3 via 9d599e0