Some searches, such as searches for private favorites or for the
status:unmoderated tag, return different results for different users.
These searches need to have their counts cached separately for each user
so that we don't return incorrect page counts when two different users
perform the same search.
This can also potentially leak private information, such as the number
of posts flagged, downvoted, or disapproved by a given user.
Partial fix for #4280.
438186a75a