Switch the license to be LGPLv2/later, with small parts still remaining as GPLv2/later, by agreement of major contributors

Reduce EnvRef:set_node() time tenfold by postponing the dayNightDiff update until it is actually needed

- Node placement code moved to Lua
- Mineral system removed (added default:stone_with_coal and default:stone_with_iron).
- MapBlock and MapNode serialization updated.
- Mapgen: Frequency of iron increased.
- node_dig callback and related changes.

Make map generator as much threaded as possible (not much benefit with current generator because of small generator chunk size (a single MapBlock))

Don't allow placing CONTENT_IGNORE with Map::setNode() because it is never useful and is only caused by bugs.