QHostInfoLookupManager prevents app from quitting if name resolution hangs

QHostInfoLookupManager connects a signal handler to `QCoreApplication::destroyed`  that waits until all threads from`QHostInfoLookupManager::threadPool` are done (see QHostInfoLookupManager::QHostInfoLookupManager()).

However, depending on the system integration, those threads may hang for up to 10 - 15sec. Thus, quitting an app would be delayed by the same amount of time.

 For example, quitting the sample app provided in attachments always hang with the following setup:

• Linux 5.4.
• Qt v5.15.0.
• Connman v1.37.
• Connman runs without --nodnsproxy, so it proxies dns resolution.
• System has no network access (system cannot reach external dns server).

Qt shouldn't rely on system level timeouts to quit an app. I would have expected Qt to force quit the threads if they are not done after a small timeout window defined by Qt.