Optimize Piece-s memory usage

[gzsombor]

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.