Support multipart copy for s3->s3 transfers

We now fully support all parallel transfers for all supported transfer types:

* local -> s3
* s3 -> local
* s3 -> s3
Support multipart copies with the "cp" command
Fix issue with float casting
Merge branch 's3-multipart-copy' into develop

* s3-multipart-copy:
  Fix issue with float casting
  Support multipart copies with the "cp" command
  Support multipart copy for s3->s3 transfers