Rudder

We don't use scala compiler to its full power. In fact, we don't even use it for simple things like unused imports.

In #11955, we did an ad hoc compilation with the interesting flags, but it is not persistent and we need to enforce them in Maven pom to be sure to not drift from the clean state.

https://tpolecat.github.io/2017/04/25/scalac-flags.html gives an overview of all the interesting flags. We won't use them all right now, because our code base it not ready :)

We will also need to be able to selectively silent some warning, in particular in the case of deprecationWarning for things that can't be replaced (easely or at all, like Thread.stop). For that, we will use the silencer plugin: https://github.com/ghik/silencer

This change will span modification on all scala projects, this ticket handle modifications in `parent-pom` one.