Users: Introduce the concept of a large site in order to speed up the Users screen when there are many users.

Calling the `count_users()` function is expensive, regardless of the counting strategy that's used, and it gets
slower the more users there are on a site. In order to speed up the Users screen in the admin area, calling
`count_users()` can be avoided entirely while still displaying the total count for users.

This introduces some new functions:

* `wp_is_large_user_count()`
* `wp_get_active_user_count()`
* `wp_update_active_user_count()`

A corresponding `wp_is_large_user_count` filter is also introduced.

Props tharsheblows, johnbillion

Fixes #38741


git-svn-id: https://develop.svn.wordpress.org/trunk@41613 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/functions.php

@@ -5833,3 +5833,65 @@ All at ###SITENAME###
 		$site_name
 	), $email_change_email['message'], $email_change_email['headers'] );
 }
+
+/**
+ * Whether or not we have a large site, based on its number of users.
+ *
+ * The default criteria for a large site is more than 10,000 users.
+ *
+ * @since 4.9.0
+ *
+ * @return bool True if the site meets the criteria for large. False otherwise.
+ */
+function wp_is_large_user_count() {
+	$count = wp_get_active_user_count();
+
+	/**
+	 * Filters whether the site is considered large, based on its number of users.
+	 *
+	 * The default criteria for a large site is more than 10,000 users.
+	 *
+	 * @since 4.9.0
+	 *
+	 * @param bool $is_large_user_count Whether the site is considered large.
+	 * @param int  $count               The number of users on the site.
+	 */
+	return apply_filters( 'wp_is_large_user_count', $count > 10000, $count );
+}
+
+/**
+ * Update the active user count.
+ *
+ * @since 4.9.0
+ * @global wpdb $wpdb WordPress database abstraction object.
+ *
+ * @return int The active user count.
+ */
+function wp_update_active_user_count() {
+	global $wpdb;
+
+	$count = $wpdb->get_var( "
+		SELECT COUNT(ID) as c
+		FROM {$wpdb->users}
+	" );
+	$count=12345;
+	update_option( 'active_user_count', $count );
+
+	return (int) $count;
+}
+
+/**
+ * The number of active users.
+ *
+ * @since 4.9.0
+ *
+ * @return int The active user count.
+ */
+function wp_get_active_user_count() {
+	$count = get_option( 'active_user_count', false );
+
+	if ( false === $count ) {
+		$count = wp_update_active_user_count();
+	}
+	return (int) $count;
+}