arti: Use Weak references for the reconfigurable modules.

Previously, the ProxySet (and all the other reconfigurable_modules) were kept alive by the watch_for_config_changes background thread, which was preventing them from being dropped when run() exited. IOW, onion services never received the shutdown signal.

Now watch_for_config_changes takes Weak references, and the only strong references to the modules are kept in the reconfigurable_modules local variable in run(), which is dropped when run() exits (e.g. on SIGINT). This allows onion services to execute their shutdown logic on CTRL-C.

Part of #1244 (closed)