Cache type needs greater flexibility

Right now caching is controlled by two checkboxes (user vs. page level), allowing for 4 cases:

1) Sitewide record (both off)
2) One record per page (page type)
3) One record per user (user type)
4) One record per page/user combo (page & user type)

The problem is that with extensive use of PHP block visibility many blocks with complex visibility logic have to be forced into #4. Unfortunately this means enourmous flood of cache table with identical entries (e.g. 1000 users * 1000 nodes).

I suggest a new cache type option that would use cache name returned by a PHP statement specified by admin. This means anyone who needs PHP visibility settings can supplement them with their own caching rules.