only buffer the first part of a large backup, stream the rest without buffering

[sbrudenell]

currently when uploading to S3, we buffer 5 GiB chunks to disk before uploading each.

It's necessary to buffer the first part as this is the only way to make the correct choice of PutObject vs CreateMultipartUpload while minimizing API usage.

however, it's probably inefficient to do this for every part. For the second part onward, we should stream the upload directly.