Create an Asynchronous progress bar

Give the progress bar its own thread and let it update the entire display asynchronously at a specified time interval. The existing implementation refreshes the progress bar for each network packet downloaded. Over multiple downloader threads and a high speed network connection this can lead to far too many redrawings of the screen. Also, each of the downloader threads will block while trying to acquire the thread mutex because another thread just retrieved a packet. While I haven't profiled it, it seems like there would be extremely high lock contention in the existing implementation. Instead, with a separate thread, we can update all the progress bar slots simultaneously at regular intervals.

Closes #90. This set of commits is a cleaner version.