Also rewrite mapgen registration for static initialization

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

Add support for notify-on-decoration
Clean up mapgen constructors
Clean up mapgen.cpp code style somewhat
Remove trailing whitespace from some files

Add BiomeManager, OreManager, DecorationManager, and SchematicManager

MapgenParams is no longer a polymorphic class, eliminating the need for messy and bug-prone reallocations.
Separation between the common and mapgen-specific parameters is now strongly defined.
Mapgen parameters objects are now properly encapsulated within the proper subsystems.

- Neatly placed all emerge related code into a new file, emerge.cpp
- Greatly cleaned up the code in EmergeThread::Thread()
- Reworked Emerge queue.  Now an actual std::queue of v3s16 block positions
- Removed the completely unnecessary map of peer ids requesting blocks