Use systematic approach to generate hash

The basic idea is to hash each class member and then mix them with xor
operation.
However the `seed` must be handled with care, it should only be
introduced once when mixing the hashes of each class member, otherwise
under some circumstances the `seed` might xor with itself and thus break
the intended effect.

src/base/bittorrent/trackerentry.cpp

@@ -162,5 +162,5 @@ bool BitTorrent::operator==(const TrackerEntry &left, const TrackerEntry &right)
 
 uint BitTorrent::qHash(const TrackerEntry &key, const uint seed)
 {
-    return (::qHash(key.url(), seed) ^ key.tier());
+    return (::qHash(key.url(), seed) ^ ::qHash(key.tier()));
 }

src/base/net/downloadmanager.cpp

@@ -348,7 +348,7 @@ Net::ServiceID Net::ServiceID::fromURL(const QUrl &url)
 
 uint Net::qHash(const ServiceID &serviceID, const uint seed)
 {
-    return ::qHash(serviceID.hostName, seed) ^ serviceID.port;
+    return ::qHash(serviceID.hostName, seed) ^ ::qHash(serviceID.port);
 }
 
 bool Net::operator==(const ServiceID &lhs, const ServiceID &rhs)