s3: re-implement multipart upload to fix memory issues
s3: re-implement multipart upload to fix memory issues

There have been quite a few reports of problems with the multipart
uploader using too much memory and not retrying possible errors.

Before this change the multipart uploader used the s3manager
abstraction in the AWS SDK.  There are numerous bug reports of this
using up too much memory.

This change re-implements a much simplified version of the s3manager
code specialized for rclone's purposes.

This should use much less memory and retry chunks properly.

See: https://forum.rclone.org/t/memory-usage-s3-alike-to-glacier-without-big-directories/13563
See: https://forum.rclone.org/t/copy-from-local-to-s3-has-high-memory-usage/13405
See: https://forum.rclone.org/t/big-file-upload-to-s3-fails/13575

ncw/rclone,ncw/rclone,ncw/rclone,ncw/rclone