Instead of having the Swift storage engine try to delete the empty chunk(s) synchronously, we simply queue them and have a worker come along after 30s to delete the empty chunks. This has a few key benefits: it is async (doesn't slow down the push code), helps deal with Swift's eventual consistency (less retries necessary) and is generic for other storage engines if/when they need this as well
Amazon S3 does not allow for chunk sizes larger than 5 GB; we currently don't handle that case at all, which is why large uploads are failing. This change ensures that if a storage engine specifies a *maximum* chunk size, we write multiple chunks no larger than that size.
