current_site should contain the network's blog_id, not the site's

A second database lookup is required to retrieve the network's
blog ID so that is_main_site() can work properly when the cache
is used to gather current_site information.

Previously, it was possible to pollute the cache with the wrong
blog_id for the network and cause confusion in the admin
Merge pull request #189 from washingtonstateuniversity/site-cache

[PR] current_site should contain the network's blog_id, not the site's