Make sure core Flink artifacts have no specific logger dependency

Flink's code is written against slf4j

To make sure users can use their custom logging framework we need to have

• no direct compile-scope dependency in any core
• a dependency in flink-dist that is not in the fat jar
• an explicit dependency in examples (to see logs when running in the IDE)
• an explicit test dependency (for logs of test execution)

All except point (1) are already fixed.