IIS advanced log importing, netscaler log importing and cs-username handling

Includes changes to the log importer that should fix issues: #6795, #4707, #5980

@quba & @mattab can you take a look and see if these changes will provide the necessary features?
No it's all good, as in fact in Piwik our metric also includes the time for client to download response. So it's consistent :+1: