Introduce a notoptions cache for site options

[duck_]

Following up on #18955.

Adding a 'notoptions' cache for site options would be consistent with normal options.

[Otto42]

+1 This should be fixed.

I found almost 30 queries per pageload on a multisite site because it was repeatedly querying for a nonexistent "ms_files_rewriting" site option key. Adding the key fixed the problem, but the notoptions cache should have been there to prevent the multiple queries in the first place.

[wonderboymusic]

First pass

[duck_]

Replying to wonderboymusic:

First pass

Uploaded 19008.2.diff​ to replace get_option() with the accidentally dropped get_site_option() in add_site_option(). Otherwise this looks good to me.

[ryan]

Looks good.

[duck_]

In 25043:

Introduce a notoptions cache for site options.

Props wonderboymusic. Fixes #19008.