Optimize Piece-s memory usage

Remove a couple of unnecessary fields from the Piece class : 
 -  offset -> can be calculated from index * pieceLength
 -  seeder -> torrent.isSeeder()
 -  bucket -> torrent.getBucket()
 -  hash -> which is equivalent with torrent.getPiecesHashes() from index*Torrent.PIECE_HASH_SIZE, so we don't have duplicate this information.