Cron Improvements with PHP-FPM in WordPress 5.1

A new change in 5.1 Beta 1 introduces a change in behaviour for cron spawning on servers running fastcgi and PHP 7.0.16+.

When available, requests to wp-cron.php will now return a repsonse immediately, before processing lengthy cron jobs, allowing the request that spawned the cron process to end without blocking regardless of the transport method used to trigger it. See #18738 for discussion and research.

This should work well on PHP-FPM installs and cause no issues on other servers. We could use some testing and reports to confirm this, since a similar method could be used in other contexts such as shutdown hooks if it proves to be safe and reliable (see #41358, #27669, #27122).

#5.1 #dev-notes