upnp: fix early crash by std::terminate

In NAP-PMP implementation, a std::thread member of
UPnPContext class is placed before member used by the thread.
This thread is run during the class initialization (ctor).
So the thread can start and access to these members when
they are not initialized.

The consequence is a std::terminate() call causing the stop
of daemon.

Fix that by moving the thread member after *ALL* used members.

Reviewed-by: Andreas Traczyk <andreas.traczyk@savoirfairelinux.com>
Change-Id: Ibb7334e653a9c80c566ffc3944c93a6ef64e0a8b