[s3][feature request] shell command to clean up stale multipart uploads

[PeterCxy]

Occasionally, S3 clients may fail during a multipart upload, leaving a number of incomplete multipart uploads behind in the .uploads directory. It would be helpful to have a command to clean these up (e.g. s3.cleanup.multipart -age 3d to clean up all incomplete multipart uploads created more than 3 days ago) to save space. Just deleting .uploads will not work because there may be ongoing multipart uploads.