Only send netinfo clock_skew to controller if an authority told us so
We were triggering a CLOCK_SKEW controller status event whenever we connect via the v2 connection protocol to any relay that has a wrong clock. Instead, we should only inform the controller when it's a trusted authority that claims our clock is wrong. Bugfix on 0.2.0.20-rc; starts to fix bug 1074. Reported by SwissTorExit.