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