All secondary user buckets should be global cache groups

[nacin]

users, user_meta, and userlogins are all global cache groups. For some reason, useremail and userslugs are not. They should be.

@dd32 confirmed they are global on WordPress.com (and probably have been forever), which is where I would have expected an esoteric bug like this would have cropped up.

[nacin]

I also found a typo in a rare code block in switch_to_blog(), a leading space in the blog-id-cache global group.

[nacin]

In 31347:

Add useremail and userslugs as global cache groups. fixes #31243.

[nacin]

In 31348:

Remove leading space from the definition of a global cache group.

This only applied in a rare situation during the switch_to_blog() process where no global groups were currently defined.

see #31243.