s3: object deletion when removing a published repo is slow

When using aptly to 'unpublish' an old repository backed by s3, we noticed that deleting the files took a very long time. After investigating we found that aptly was issuing delete command on files one-by-one. For a normal Debian repository this can take a long time.

S3 protocol can support multi-object deletion, which allows for bulk deleting objects and greatly speeds up this round trip.