Stem should invalidate GETCONF cache

Stem caches the result of GETCONF queries, but only updates its cache if SETCONF is done through that Controller. There could be other controllers that alter tor's configuration out from under us.

Now that we have CONF_CHANGED events this should be easy enough to fix (#1692 (moved)). The Controller class should add its own listener for CONF_CHANGED events that does cache updates similar to set_options().