Ensure pageviews are added to date of recording in case of scheduled event failures

[dannyvankooten]

If WP Cron is somehow blocked (ie another plugin throwing a fatal error), then any pageviews in the buffer file will end up being processed at a potentially later date than when they were recorded, and thus added to the wrong date.

It would be nice if we could just backfill pageviews to their correct date. It would still be nice to not have to repeat the date for every row in the buffer file though, so perhaps a different buffer file per day would be a good solution? Or something like a special row which indicates the date...