This trait brings log level handling to the EventStream: it reads the log
levels for the initial logging (StandardOutLogger) and the loggers & level
for after-init logging, possibly keeping the StandardOutLogger enabled if
it is part of the configured loggers.
Maps Subscribers to Classifiers using equality on Classifier to store a Set of Subscribers (hence the need for compareSubscribers)
Maps Events to Classifiers through the classify-method (so it knows who to publish to)