Introduce river mapfile generation (WARNING: Changes in config xml files required!). On startup the RiverMapfileGeneratorStarter is called by the server app. The starter invokes a call to RiverMapfileGenerator.generate() which reads all available rivers from the database and generates layers for each river (in theory...uncompleted) in a river.map file. The old MapfileGenerator which generates layers that have artifact dependencies is now in ArtifactMapfileGenerator.