Make beforeLayergroupCreate to return a datasource with different
authentication for the different layers.
- Named layers will get access to private tables in case it's needed
Changes in MapConfigNamedLayersAdapter:
- It will retrieve the dbAuth params only if named layers are present so
there is no extra overhead for normal layers
- Rename queue function signature from `callback` to `done` so it is easier
to follow the code
Add several tests to validate `named` layers authentication
