Replace PutObject with composed struct

[msiebuhr]

We have a frequent use for getting some file contents and knowing its size up-front; in the client when we want to upload things and in the server when we return things.

A rough sketch:

package main

import (
	"fmt"
	"io"
	"os"
)

type SizeReader interface {
	io.Reader
	Size() int64
}

type SizeFile struct {
	*os.File
}

func (sf *SizeFile) Size() int64 {
	stat, _ := sf.Stat()
	if stat == nil {
		return 0
	}
	return stat.Size()
}

// Add things that makes sized random readers, network, ...

func main() {
	x, err := os.Open("./test.go")
	if err != nil {
		panic(err)
	}
	sf := &SizeFile{x}
	fmt.Println(x, err, sf.Size())
}

Would also be useful for the streaming put/get interfaces...