Use *multipart.Reader to stream file parts instead of storing temporary FileHeaders in memory

[ciehanski]

A possible implementation is below:

func uploadMR(mr *multipart.Reader, writer io.Writer) error {
	for {
		part, err := mr.NextPart()
		if err == io.EOF {
			break
		}
		if part.FileName() == "" {
			continue
		}
		var count int
		chunk, err := Allocate(256)
		if err != nil {
			return err
		}
		for {
			count, err = part.Read(chunk)
			if err == io.EOF {
				break
			}
			if _, err := writer.Write(chunk[:count]); err != nil {
				return err
			}
		}
	}
	return nil
}

However, I may not need to read chunks of the part considering the part will already be a chunk as its being received by the HTML form. Will need to create benchmarks, this could drastically improve upload performance which is the largest bottleneck currently due to the HTML form contents being stored in memory, sanitized and then re-written back to memory. Would like to see the use of *multipart.Reader before any major release.