Change logging to SLF4J

[patrick-skowronek]

In the pom.xml there is a comment stating "TODO: Change logging to SLF4J ???"

Should we do that or change the loggin from log4j 2 to slf4j or chage it to another lib?
@alexanderkiel @torbrenner @TasDeniz

[alexanderkiel]

slf4j is a logging API which is widely used in Java. I would update all apps and libs to use that API. After that, I would use slf4j's own logging backend logback instead of log4j.