Fix wpo_last_page_cache_preload timestamp

I would modify the file ‘class-wp-optimize-page-cache-preloader.php’ in order to increase the default value of the constant ‘allowed_time_difference’ since 10 minutes is not enough… let me explain.

If I check the lifespan to a certain value and then check the preload scheduler equal to the lifespan and if the preload time is higher than 10 minutes, then, the next execution of the event ‘wpo_page_cache_schedule_preload’ in the scheduler will check the ‘time_since_last_preload’ and compare with the ‘$page_cache_lifespan – $allowed_time_difference’ and it will happen that ‘time_since_last_preload’ will be always lower than ‘$page_cache_lifespan – $allowed_time_difference’ if preload execution time is highet than ‘allowed_time_difference’ so it will skip preload one every two executions.

I don’t know if I explained myself properly, but after many checks on my website, I had to manually increase ‘allowed_time_difference’ because the preload in my website takes almost 20 minutes to be complete.

Other better solution should be to mark the ‘wpo_last_page_cache_preload’ field on the database with the start time of the preload instead of the end time of the preload. In this case the issue detailed abover will not happen.