User stats should have its own generator.

The 4.2.0 release already uses the cached_stats generator, so we should not add to it lest we break/complicate the upgrade path.  This takes the modifications to that generator and breaks them out into their own generator, which will enable adopters to upgrade cleanly from 4.2 to 4.3.
Merge pull request #792 from projecthydra/fix_migrations_for_43x_release

User stats should have its own generator.