Change QtNetwork to use more efficient QHttpHeaders iteration

Parts of QtNetwork has been internally ported to use the new QHttpHeaders class.

Nowadays the QHttpHeaders a more effective iteration API (size(), valueAt(), nameAt()) which, to my recollection, weren't available when the porting was made.

That iteration API should be used instead of the "toListOfPairs()" and "toMultiMap()" and "toMultiHash()",
(whichever are applicable).