QSslSocket should go into UnencryptedMode after setSocketDescriptor is called

With QTcpSocket and QLocalSocket one can reuse these objects by just calling setSocketDescriptor again, which proved to be faster than creating the objects on high traffic servers (case of Cutelyst's), but QSslSocket::startServerEncryption() fails because it's internal mode is not UnencryptedMode anymore thus leading to a warning without any API feedback to know that we failed to start the encryption.

IMO the proper solution would be to reset this value when setSocketDescriptor is called.