Reduce the number of calls to QThread::idealThreadCount() in QtConcurrent.
Reduce the number of calls to QThread::idealThreadCount() in QtConcurrent.

This results in a syscall and is very slow. Make the call once and cache
the value.

Task-number: 244718
Reviewed-by: TrustMe

igor-sfdc/qt-wk,RLovelett/qt,pruiz/wkhtmltopdf-qt,radekp/qt,RLovelett/qt,igor-sfdc/qt-wk,radekp/qt,KDE/qt,radekp/qt,RLovelett/qt,radekp/qt,radekp/qt,KDE/qt,RLovelett/qt,igor-sfdc/qt-wk,igor-sfdc/qt-wk,igor-sfdc/qt-wk,pruiz/wkhtmltopdf-qt,KDE/qt,radekp/qt,KDE/qt,KDE/qt,RLovelett/qt,RLovelett/qt,KDE/qt,igor-sfdc/qt-wk,radekp/qt,pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,RLovelett/qt,radekp/qt,RLovelett/qt,igor-sfdc/qt-wk,pruiz/wkhtmltopdf-qt,pruiz/wkhtmltopdf-qt,pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,RLovelett/qt,pruiz/wkhtmltopdf-qt,KDE/qt,pruiz/wkhtmltopdf-qt,igor-sfdc/qt-wk,radekp/qt,RLovelett/qt,RLovelett/qt,igor-sfdc/qt-wk,KDE/qt,pruiz/wkhtmltopdf-qt,radekp/qt,pruiz/wkhtmltopdf-qt,pruiz/wkhtmltopdf-qt