Use/add stone_surface_max_y to speed-optimise/guide dungeon generation
MgV7: Don't let mountain terrain chop dungeons at mapchunk borders
Make mountain terrain update stone_surface_max_y for caves in mountains

Also rewrite mapgen registration for static initialization

Add minetest.get_mapgen_params()
Deprecate minetest.register_on_mapgen_init()

Fixes #1807: When the server list finishes downloading, the local server
list resets in certain conditions

Also set default height_min/height_max to -31000 and 31000,
respectively, for ore and biomes

Biome API: Add shore top and shore filler nodes, underwater node, water top node. Add water top depth and shore height parameters. Remove water dust node