negative caching in dl_link; key len limit; cache has_storage (#1440)
remove errant prints (#1440)
manually create cache directories (#1440)
cleanly put cache dirs under /mnt on prod (#1440)
Merge pull request #1882 from Stanford-Online/sef-1440-s3-caching

negative caching in dl_link; key len limit; cache has_storage (#1440)