solve deadlock when QNAM is deleted while http delegate thread is still running

The use of QSharedPointer<QNetworkSession> was under the assumption that QNAM would have a longer lifetime than the http thread(s) it creates.
In the qabstractnetworkcache test case, this isn't true.
It deadlocks when the http thread tries to delete the QNetworkSession - as the delete is called from the wrong thread.

QNAM is a local variable in a test case, so it is destructed immediately on a test failure.