Optimise chunk set (#4260)

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

src/ChunkData.cpp

@@ -113,7 +113,7 @@ void cChunkData::Assign(cChunkData && a_Other)
 		return;
 	}
 
-	if (&m_Pool != &a_Other.m_Pool)
+	if (m_Pool != a_Other.m_Pool)
 	{
 		// Cannot transfer the memory, do a copy instead
 		const cChunkData & CopyOther = a_Other;