adding explicit list of cacheable documents

__subclasses__ lookup didn't seem to work.
1: too magical (my bad)
and 2: celery bootstrap doesn't bootstrap the way we expect, so the assumptions for all the subclasses to show up wasn't right. this correctly now bootstraps and gets all generational caching bits working now in a celeyr environment.
Merge pull request #1502 from dimagi/cache_checkpoint

adding explicit list of cacheable documents