One thing fairly common to the cache groups is a block of code to look to see when the cache was last changed, and if there isn't one, to set it for the current microtime(). It appears in 8 different places in core. This adds a new helper `wp_cache_get_last_changed` to DRY things up a bit.
Since `wp-includes/cache.php` isn't guaranteed to be loaded, this new function is in `wp-includes/functions.php`
Props spacedmonkey, desrosj.
Fixes#37464.
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Provides a consistent way to query `$wpdb->site` for `WP_Network` objects based on domain, path, network ID, and (main) site ID.
Introduces and uses update_network_cache() and _prime_network_caches() to maintain a cached list of WP_Network objects for use in multiple queries.
Props flixos90.
See #32504.
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