speed up the loading of the world

[maxilevi]

after re-reading the code I realized a few things can be done to achieve this:

• profile the code and find which queue is the bottleneck (the one that generated the chunk or the one that builds it).

• optimize the generation queue by skipping the blocks that won't be used when building the chunk

• farther chunks can be built with lod and only generating a few parts, updating them as they come closer.

• avoid deleting and creating new chunks on ChunkLoader.cs

[maxilevi]

this probably will improve #3