Passes the file handle created in main() directly into the s3manager.
Passes the file handle created in main() directly into the s3manager.

- Changes the type signature of the file handle to io.ReadSeeker to
  take advantage of optimizations described in the AWS documentation at
  https://github.com/aws/aws-sdk-go/wiki/S3Manager-Uploader.

[#105489488](https://www.pivotaltracker.com/story/show/105489488)

pivotal-golang/s3cli,pivotal-golang/s3cli,pivotal-golang/s3cli,pivotal-golang/s3cli