Do not use direct starting of HTTP request if in a slot that was triggered by same QNAM

(talk to Markus about this)

We seem to have this code already:

void QHttpNetworkConnectionChannel::allDone()
[...]
    bool emitFinished = reply->d_func()->shouldEmitSignals();
[...]
    // queue the finished signal, this is required since we might send new requests from
    // slot connected to it. The socket will not fire readyRead signal, if we are already
    // in the slot connected to readyRead
    if (emitFinished)
        QMetaObject::invokeMethod(reply, "finished", Qt::QueuedConnection);
[...]

which is bad worse for performance since it means that we will always go via the event loop when a HTTP request is finished.
What we should do is emit finished() directly and while emitting set something like QNetworkAccessManagerPrivate.isEmitting=true.
Then when a new get() is requested in the QNAM we would check the isEmitting first and will only go the "direct sending" way if it is false.