Sometimes we need to update a _lot_ of ACLs on S3 (such as when secure media
is enabled), and since it takes ~1s per upload to update the ACL, this is
best spread out over many jobs instead of having to do the whole thing serially.
In future, it will be better to have a job that can be run based on
a column on uploads (e.g. acl_stale) so we can track progress, similar
to how we can set the baked_version to nil to rebake posts.
Martin Brennan