Multisite load: Cache the main site lookup query.

props jeremyfelt.
see #27003.


git-svn-id: https://develop.svn.wordpress.org/trunk@27664 602fd350-edb4-49c9-b593-d223f7449a82
Use wp_cache_add() instead of wp_cache_set() when we don't want an unconditional set.

props jeremyfelt.
see #27003.


git-svn-id: https://develop.svn.wordpress.org/trunk@27665 602fd350-edb4-49c9-b593-d223f7449a82