oonibackend fails to start tor when asking it to drop priviledges

The changes introduced to fix compatibility with twisted >= 13.1 have made it not possible to use oonibackend as a bouncer and collector when dropping priviledges.

This can be reproduces by setting main->uid and main->gid to something different than the current using and running ooniprobe as root.

I believe the issue has to do with the fact that the startTor function does not finish running before priviledges are dropped and therefore it will at a certain point fail to communicate to the tor process and it will never trigger the successful bootstrap.