LibraryFileDownloadCount breaks Librarian garbage collection

We are adding a new table, LibraryFileDownloadCount, to track download counts of stuff served from the Librarian so we can draw pretty graphs.

The Librarian garbage collector needs to be updated to ignore this table. At the moment it will detect all the LibraryFileAlias links and think the LFA is valid, so if a file has ever been downloaded it will never be garbage collected.

Skipping reference checks on this table is all that is required - the constraint is ON DELETE CASCADE so referring rows will be automatically removed when the LibraryFileAlias is removed.