[S3] cbbackupmgr should not use a 'bufio.Reader' for the upload pool

What's the issue?
We currently wrap the reader we pass to the 's3manager.Upload' function with a 'bufio.Reader', this is fine, however, it's not seekable; this leads to excessive memory usage since instead of using 'io.SectionReader's it read the data into memory.

What's the fix?
We don't wrap the file reader with a 'bufio.Reader'.