Port QDnsLookupThreadPool to Q_APPLICATION_STATIC

The QDnsLookupThreadPool is-a Q_OBJECT subclass of QThreadPool just so it can listen to the QCoreApplication::destroyed() signal and shut itself down. All this scaffolding has recently been abstracted into a macro Q_APPLICATION_STATIC. As a result, we could save one QObject subclass (together with RTTI etc often saving 1KiB of TEXT), because the Q_A_S payload object need not ba-a QObject, it can aggregate intead (has-a QObject).