Fixed stop downloading bug in 0.0.18

Finally, I have found the problem of BitTornado 0.3.9b. It will cancel requests that are not being responsed within snub time. After that it will choke itself. It is so hard to get unchoke from other peers at this point.

I just commented this code and BTQueue backs to the fastest client right now. I will release it as 0.0.19 very soon.