Users: Introduce the concept of a large site to single site installations.

Currently in WordPress multisite there is a concept of large networks. The function `wp_is_large_network` is used to determine if a network has a large number of sites or users. If a network is marked as large, then 
expensive queries to calculate user counts are not run on page load but deferred to scheduled events. However there are a number of places in a single site installation where this functionality would also be useful, as 
expensive calls to count users and roles can make screens in the admin extremely slow.

In this change, the `get_user_count` function and related functionality around it is ported to be available in a single site context. This means that expensive calls to the `count_users` function are replaced with 
calls to `get_user_count`. This change also includes a new function called `wp_is_large_user_count` and a filter of the same name, to mark if a site is large.

Props johnbillion, Spacedmonkey, Mista-Flo, lumpysimon, tharsheblows, obenland, miss_jwo, jrchamp, flixos90, macbookandrew, pento, desrosj, johnjamesjacoby, jb510, davidbaumwald, costdev. 
Fixes #38741.



git-svn-id: https://develop.svn.wordpress.org/trunk@53011 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/schema.php

@@ -1267,6 +1267,7 @@ We hope you enjoy your new site. Thanks!
 		'subdomain_install'           => $subdomain_install,
 		'global_terms_enabled'        => global_terms_enabled() ? '1' : '0',
 		'ms_files_rewriting'          => is_multisite() ? get_site_option( 'ms_files_rewriting' ) : '0',
+		'user_count'                  => get_site_option( 'user_count' ),
 		'initial_db_version'          => get_option( 'initial_db_version' ),
 		'active_sitewide_plugins'     => array(),
 		'WPLANG'                      => get_locale(),