Optimize performance of QNetworkAccessManager

Using QNetworkAccessManager to load files is very slow compared to using RFile. Performance of QNetworkAccessManager is important as QtWebKit uses it to load files and hence suffers a substantial loss of performance compared to using native API. Proposal is to benchmark why QNetworkAccessManager is slower than native API and to optimize performance if possible.

Copy/Paste from findings when benchmarking QNetworkAccessManager against RFile

Some tests show that Qt network access manager is consistently ~50-100% slower in performance compared to using Symbian file access performance. Test code is available if it should be needed.