Step names must be unique #3757

[FBibonne]

the names of different steps in a job must be different

Sloves #3757

A few comments :

• pom is updated to have a have a version of slf4j-api corresponding to simple-slf4j
• unicity of names is checked when starting the job , just before calling listeners because step names may be known only in JobScope
• in FlowJob.findStepsThrowingIfNameNotUnique, I suggest to remove the else block when state is instance of StepHolder : it is never executed