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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git-svn-id: http://core.svn.wordpress.org/trunk@37835 1a063a9b-81f0-0310-95a4-ce76da25c4cd
