Cron API: Make wp-cron.php non-blocking where possible.

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

Built from https://develop.svn.wordpress.org/trunk@44488


git-svn-id: http://core.svn.wordpress.org/trunk@44319 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-cron.php

@@ -18,6 +18,11 @@
 
 ignore_user_abort( true );
 
+/* Don't make the request block till we finish, if possible. */
+if ( function_exists( 'fastcgi_finish_request' ) && version_compare( phpversion(), '7.0.16', '>=' ) ) {
+	fastcgi_finish_request();
+}
+
 if ( ! empty( $_POST ) || defined( 'DOING_AJAX' ) || defined( 'DOING_CRON' ) ) {
 	die();
 }

wp-includes/version.php

@@ -13,7 +13,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '5.1-alpha-44487';
+$wp_version = '5.1-alpha-44488';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.