Something needs to hold a non-weak ref to BridgeDescMgr.

I think that right now nothing actually holds a strong reference to our BridgeDescMgr: when we get into update_desired_descs, we have self.bridge_desc_provider.is_some() == true, but self.bridge_desc_provider.as_ref().and_then(Weak::upgrade).is_some() == false. To me, that says that nobody has actually kept a strong reference.

It would match our behavior elsewhere if we were to store the Arc<·> in TorClient. What do you think?