fix s3 multipart upload to account for more than 1000 parts
fix s3 multipart upload to account for more than 1000 parts

S3 multipart file uploads that have more than 1000 chunks fail due to `boto`
paginating the results of `multipart_upload.get_all_parts()` to 1000.  The
`boto` docs recommend using the `MultiPartUpload` object as an iterator instead
so that pagination happens automatically.

http://boto.cloudhackers.com/en/latest/ref/s3.html#module-boto.s3.multipart

ohmu/pghoard,hnousiainen/pghoard,ohmu/pghoard,hnousiainen/pghoard,saaros/pghoard