Speedup loading of partial databases from cache

[hagenw]

When loading a small sub-set, like the test set, of a large database the second time (which means from cache) it can be slow as we have to execute all lines of code that look for missing tables and media files as the database is not marked as complete in cache.
Maybe we could avoid this by not only marking if a database is complete, but also the string that was passed on to media and tables, to provide faster loading of a database if the same sub-set is requested again.