Stem should invalidate exit_policy cache for DNS_USELESS event

As originally noted in #25423 (closed), a number of ControlPort events indicate that tor has changed its exit policy; stem should use these to invalidate the exit_policy cache. All events mentioned in #25423 (closed) appear to be implemented except for the [event].

(So this ticket is for that.)

==== Severity/priority This event should be very rare, and there are probably a number of other things consumers would want to do if this event occurred, that are out of scope for this ticket. Overall I believe this is best categorized as Low/Minor.