should use dcplib for s3 multipart constants

https://github.com/HumanCellAtlas/data-bundle-examples/blob/e89afc2fa9cfcdb8f8d3b642c04245311839cd1a/bin/bundle_tools/s3.py
https://github.com/HumanCellAtlas/data-bundle-examples/blob/5e3849874300f4461f0001a479a449cc9241cf21/bin/stage_and_store.py