no way to await TorClient being fully stopped after dropping it

in onionmasq, when using PTs, we sometime get this error: Configuration requires exclusive access to shared state, but another instance of Arti has the lock: bridges configured.

I believe it happens when the user stops the VPN, and then starts it again, but the previous TorClient wasn't fully stopped yet. We'd like to have something akin to Channel::wait_for_close, but for the entire TorClient, which we can await to make sure we released the lock on the data dir.

related to onionmasq#97 (closed)