tuf code might have race conditions

[groob]

I'm working on #13 to add a Download method which downloads a specific target.
In order for the download method to work, it must call refresh() first to update all the local metadata from Notary. The refresh method works by updating pointers on a struct, and is also called by the updater loop which runs on a timer.
This new way of using the code will result in race conditions.

	root, err := rs.refreshRoot()
	if err != nil {
		return "", errors.Wrap(err, "refreshing root")
	}
	// cache the current root
	rs.root = root
	timestamp, err := rs.refreshTimestamp(root)
	if err != nil {
		return "", errors.Wrap(err, "refreshing timestamp")
	}
	rs.timestamp = timestamp