wp_get_active_network_plugins() can generate a needless query

[nacin]

In #18955, we stopped caching the default value when get_site_option() was called on an option that does not exist.

This introduces a situation where active_sitewide_plugins may be queried for on every pageload.

1. active_sitewide_plugins should be instantiated in populate_network() and added to upgrade_network().

2. wp_load_core_site_options() should be set up to cache as false any keys it misses. As this only works when an external object cache is not at play, it basically becomes a rudimentary not-options cache. (See #19008.)

Point 1 is for 3.3. Point 2 is a general nice-to-have.

[ryan]

Handles point 1.

[nacin]

Patch from ryan addresses point 1. Due to how get_site_option() checks for null or false against $value coming from the cache, point 2 isn't obtainable for now.

[ryan]

Bump db version

[ryan]

In [19470]:

Initialize active_sitewide_plugins during network install and upgrade. fixes #19385