made the logger more configurable

Made the logger listener more configurable (see #924.)
@DerManoMann Symfony provides lots of supported extension points, and we add more when being requested to provide more extension points. but switching things to protected means that everything is an extension point, which means we cannot change anything anymore in later releases without breaking the extension points.
Inheritance is not the only way to extend code. And it is often not the cleanest one.