Closes#1244
Initially I was just going to add the cChunkData to cSetChunkData but profiling revealed
that the copying wasn't even the biggest slowdown. Much more time was being spent in
cChunk::CreateBlockEntities and cChunk::WakeUpSimulators than was in memcpy so I've made
those significantly faster as well.
Optimisations performed:
* cSetChunkData now stores blocks in a cChunkData object
* cChunkData objects can now perform moves even if they are using different pools
* cChunk::CreateBlockEntities now iterates in the correct order and only over present chunk sections
* Similarly for cChunk::WakeUpSimulators
* cSetChunkData::CalculateHeightMap now shortcuts to the highest present chunk section before checking blocks directly
