Possible easier to use top-level API

[bassosimone]

It seems the current API based on channels is not very comfortable to use. A better design would probably be to expose a blocking API with a callback, for example:

func (c *Client) Run(ctx context.Context, fn func(m *spec.Measurement)) (summary Summary, err error) {
  var ch chan spec.Measurement
  ch, err = c.StartDownload(ctx)
  if err != nil {
    return
  }
  for m := range ch {
    // TODO: fill the summary
    fn(m)
  }
  ch, err = c.StartUpload(ctx)
  if err != nil {
    return
  }
  for m := range ch {
    // TODO: fill the summary
    fn(m)
  }
  return
}

This may or may not be combined with a summary. Note that this API does not cause data races because the callback is called in the same thread context of the blocking Run call.