Site health:

- Prevent fatal errors from timeouts on the Tools => Site Health => Info tab.
- Use the `get_dirsize()` and `recurse_dirsize()` functions to calculate directory sizes. The results are cached.
- Introduce "timeout protection" in `recurse_dirsize()`.

Props pento, Clorith, xkon, afercia, jeremyfelt, azaozz.
Fixes #46645.
Built from https://develop.svn.wordpress.org/trunk@45104


git-svn-id: http://core.svn.wordpress.org/trunk@44913 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/default-constants.php

@@ -29,6 +29,11 @@ function wp_initial_constants() {
 	define( 'TB_IN_BYTES', 1024 * GB_IN_BYTES );
 	/**#@-*/
 
+	// Start of run timestamp.
+	if ( ! defined( 'WP_START_TIMESTAMP' ) ) {
+		define( 'WP_START_TIMESTAMP', microtime( true ) );
+	}
+
 	$current_limit     = @ini_get( 'memory_limit' );
 	$current_limit_int = wp_convert_hr_to_bytes( $current_limit );