Further simplify circuit state change observation

[jcorbin]

• no event emitter at all, just inject logger and statsd
• less checking necessary, and more string re-use

NOTE: since we no longer involve serviceProxy.extendLogInfo or channel.extendLogInfo, the following log fields have been removed from circuit state change logs:

info.affineServices         = Object.keys(self.exitServices);
info.channelDestroyed       = self.destroyed;
info.channelDraining        = self.draining;
info.channelListened        = self.listened;
info.channelListening       = self.listening;
info.circuitsEnabled        = self.circuitsEnabled;
info.hostPort               = self.hostPort;
info.minPeersPerRelay       = self.minPeersPerRelay;
info.minPeersPerWorker      = self.minPeersPerWorker;
info.partialAffinityEnabled = self.partialAffinityEnabled;
info.rateLimiterEnabled     = self.rateLimiterEnabled;

I think this is okay and that we don't need these fields, reviewers please double check.

r @raynos @kriskowal @rf

[kriskowal]

The lost log fields are okay.

[kriskowal]

:ok: