Migrate cache to be disk-based instead of database based, this allows for full caching as until now were limited by such things as maximum blob length of maximum database packet sizes.

Added an migration script which needs to be tested.

Added an Moderated-ringbuffer, which basically contains the spots which are recently have been moderated. This allows for much more efficient spot retrieval when both prefetch_nzb, prefetch_image and 'buggy'-nttp option has been enabled.