arti config watch: Pass a Runtime, not a TorClient

I found this confusing in !2009 (merged), where the variable is unused on Windows: I wondered if this meant our config-reloading only worked on Unix, since "obviously it would need the TorClient".

But actually the TorClient was only used in this argument for its runtime. The actual reconfiguration of the TorClient is doen by it being in one of the ReconfigurableModule's.

Change the type of the argument for clarity.

Assigning review to @nickm who has already seen !2009 (merged).