Add page load caching to get_users_of_blog()

[yoavf]

Theme authors often use get_users_of_blog() in various area to style/present blog authors. Unfortunately, the query resulting from this function isn't cached, and can be quite expensive in a large multisite environment.

Proposed patch adds simple caching to get_users_of_blog()

[Denis-de-Bernardy]

The cache also needs to be invalidated.

[westi]

This cache should only last for the page load so doesn't need invalidation.

It just needs registering as non-persistent in the right places with wp_cache_add_non_persistent_groups

[westi]

Probably should just be in the counts group in fact

[yoavf]

Rethinking this:

get_users_of_blog() returns the complete users objects for every users, yet scenarios where only user ids/nicenames/email are required, are the reason I opened this ticket in the first place.

A better solution would probably be to have a function that can return specific columns of the user table.
Does that make sense?

[yoavf]

Irrelevant, get_users_of_blog() has been deprecated in favor of get_users()
See #15053