This commit rewrites the procedure that is responsible for light
updating.

this commit
-provides iterative solutions for unlighting and light spreading
-introduces a new priority queue-like container for the iteration
-creates per-node MapBlock caching to reduce retrieving MapBlocks from
the map
-calculates with map block positions and in-block relative node
coordinates
-skips light updating if it is not necessary since the node's new light
will be the same as its old light was

This commit modifies the liquid transforming procedure to light and
unlight nodes instead of whole map blocks.