Improve findbugs-slf4j misuse of slf4j API

The goal of this issue is to upgrade to newer findbugs-slf4j (or fork and mvn deploy to ch.vorburger GAV, if needed because upstream project is unresponsive in a 2-3 weeks), with:

• https://github.com/KengoTODA/findbugs-slf4j/pull/72
• https://github.com/KengoTODA/findbugs-slf4j/issues/69
• http://blog2.vorburger.ch/2018/03/slf4j-logging-api-miss-use-detection.html

NB that the findbugs-slf4j is now all over the place ;-( and already duplicated in:

1. odlparent
2. infrautils
3. netvirt
4. genius