Request random pieces if all selections have been met

[mafintosh]

In order to be better swarm citizens we should probably think about changing piece strategy in all critical pieces have been downloaded.

A simple way of doing this would be do fetch random pieces if the selection array is empty.

[asapach]

"Rarest first" is also a good strategy: http://www.bittorrent.org/bittorrentecon.pdf

[transitive-bullshit]

I already implemented this for bittorrent-client here if anyone wants to port back over to torrent-stream. Note that bittorrent-client's internal piece selection logic is just a cleaned up, more modular version of torrent-stream's, so the port should be relatively straightforward.

On a side note, we really need to set a gameplan for better coordination between these two projects.

[transitive-bullshit]

Also note that rarest first is essentially equivalent to random selection in a healthy torrent but performs significantly better in an unhealthy torrent, so there's no reason not to go with rarest first over random.