This should make cron spawning faster by ensuring requests to wp-cron.php return immediately regardless of transport method. It is enabled only on recent PHP versions with fastcgi, due to historical bugs and availability of `fastcgi_finish_request()`. This needs testing on a range of platforms, to help determine if it's safe to use in other contexts also.
Props vnsavage, johnbillion, jnylen0.
See #18738, #41358
git-svn-id: https://develop.svn.wordpress.org/trunk@44488 602fd350-edb4-49c9-b593-d223f7449a82