TODO

 * Make use of sparse file regions in download data for faster hashing. This is available as whence 3 and 4 on some OSs?
 * When we're choked and interested, are we not interested if there's no longer anything that we want?
 * dht: Randomize triedAddrs bloom filter to allow different Addr sets on each Announce.
 * data/blob: Deleting incomplete data triggers io.ErrUnexpectedEOF that isn't recovered from.
 * Handle wanted pieces more efficiently, it's slow in in fillRequests, since the prioritization system was changed.