QNetworkAccessManager hoards sockets after QNetworkReply instances are destroyed

If a lot of QNAM request are performed to a different hosts (for example, for all 192.168.1.0 network), application may reach default 1024 open file handle limit on Linux, as it seems that QNAM keeps sockets even after all QNetworkReply instances are destroyed.

It does free sockets when QNAM itself is destroyed, but if QNAM instanced is held for all application lifetime (taken from QQmlEngine::networkAccessManager() or created single instance for reuse manually), it produces undesired errors when file limit is reached.

See attached example application to reproduce.