Use faster hash function

qHash(QString) will need to hash/loop through all the data while the new
code will only need one memcpy() and a few bit manipulations.

src/base/bittorrent/infohash.cpp

@@ -89,5 +89,9 @@ bool BitTorrent::operator!=(const InfoHash &left, const InfoHash &right)
 
 uint BitTorrent::qHash(const InfoHash &key, const uint seed)
 {
+#if (LIBTORRENT_VERSION_NUM < 10200)
     return ::qHash(static_cast<QString>(key), seed);
+#else
+    return ::qHash((std::hash<lt::sha1_hash> {})(key), seed);
+#endif
 }