Symbian socket engine: Finalize QHostInfo port

• Make sure the QSharedPointer<QNetworkSession> propagates from QAbstractSocket to the DNS lookup (search for qt_qhostinfo_lookup call)
• Use the RHostResolver factory function that Shane did
• Test thread safety (using QHostInfo from multiple threads at the same time)
• debug tst_QHostInfo::raceCondition(WithCache), fix the test if it's broken or the code otherwise

Niceties (not blockers):

• Use asynchronous version of RHostResolver::Next for asynchronous lookups
• de-duplicate common code in async and sync API implementations
• integrate with qt_QHostAddressFromTInetAddr in qnetworkinterface_symbian.cpp (for converting addresses)