Fire a circuit status control port event when circuit isolation changes.
Followup from legacy/trac#17086 (closed).
Currently trying to track circuit isolation status via control port events is non-trivial/impossible without polling due to predictive circuits and other non-SOCKS initiated circuits being used to service SOCKS requests.
The current behavior is correct according to the control port spec, since the username/password are only provided when the circuit was initiated via a SOCKS request, and is silent about
BUILT circuits being used to immediately service a request.
This should be easy to do, but requires a control spec change and is an enhancement so it's a 0.2.8.x thing at the earliest.