Single router refactoring

This PR removes the logic to create a router for each version splitted by the first part of the URI (e.g. `/v1`). This also makes it possible to serve middlewares on the root URI with a welcome message.
We want a welcome middleware to get information of a service on a known path. Here we have a representative example, that should IMHO say something like "This is welcome example 0.1.0".