Scheduler for Preload [feature request]

Create an option to set the Preload using a scheduler similar to the one that is available for scheduling garbage collection.

Preloading all of a site takes time, and the time for the next preload is set to start xxxx amount of minutes after the last one _finished_. Sometimes it takes longer than other times to complete. If I set it to preload – for example – every 1440 minutes, the start time will creep ahead a little bit each day. What started in the middle of the night, for example, will eventually end up running in mid-morning sometime.

It would be great if users had the option to run the preloading every day or every week, etc, starting at a particular time instead.

Start by locating the existing scheduler used for garbage collection and compare its behavior with the current preload timing described in the issue. Determine how a recurring daily or weekly schedule with a fixed start time should interact with preload completion; done means users can configure that schedule and preloads no longer drift based on completion time.

Written by the indexing model from the issue text.