Map generator refactor

the intentions/plans are:
1) fix the 'single use' cave map generator to work in mp.
we fix this by splitting the cave_map_generator into 
cave_map_generator and cave_map_generator _job from which the later is a temporary object used during generate_scenario/map

2) allow to pass a (platform independent) seed to all map generators, this is not completely implemented yet. I implemented this for the cave_map_generator_job and it was already implemented for the yamg. Is still needs to be implemented for the default_map_generator