Adjustable number of levels in liquids

from 1 to 63
1 - special for sand, gravel - stay solid, disallow build sand pillars 
2 - very fast flood
8 - current default
63 - very smooth and long-range flow, more cpu use
can be different for various liquids (like lava=4, water=20)
usage: 
minetest.register_node("default:water_flowing", {
...
paramtype2 = "leveled",
leveled=42,
...

also it removes buggy "range"

_game changes: https://github.com/proller/minetest_game/tree/liquid63