WP Super Cache: add speed setting to preload process

The preload process was slowed down in the last release due to problems it created on very cheap hosting.

It should be more flexible by adding a form where the site owner can:
* Choose how many requests to make per job.
* The pause between requests.
* The pause before the next job.

It should have a "RESET" button for anyone who messes things up too much.

Preload is done sequentially, so increasing the number of requests shouldn't be a problem for most sites.

The job is limited by the PHP timeout limit. That is reset by using `set_time_limit()` but some servers will still terminate the process before it finishes. The preload process may need extra checks and scheduled maintenance jobs to ensure the process continues running if a user picks numbers that are too large.