Improve "info hash" handling

Define "torrent ID" concept, which is either a SHA1 hash for torrents of version 1,
or a SHA256 hash (truncated to SHA1 hash length) for torrents of version 2.
Add support for native libtorrent2 info hashes.

src/base/bittorrent/tracker.h

@@ -102,6 +102,6 @@ namespace BitTorrent
         Http::Request m_request;
         Http::Environment m_env;
 
-        QHash<InfoHash, TorrentStats> m_torrents;
+        QHash<TorrentID, TorrentStats> m_torrents;
     };
 }