latency numbers don't measure reading the body

[stevej]

After talking with oliver, we noticed that throughput was impacted by large responses but that latency reported wasn't.

We should move measurement of elapsed down to after the body is read

    start := time.Now()
    response, err := client.Do(req)

    elapsed := time.Since(start)
    if err != nil {
        received <- &MeasuredResponse{0, 0, 0, false, err}
    } else {
        sz, _ := io.Copy(ioutil.Discard, response.Body)
        response.Body.Close()
        received <- &MeasuredResponse{uint64(sz), response.StatusCode, elapsed.Nanoseconds(), false, nil}
    }