Tnt: Various optimisations

Pass nodename to tnt.burn function where possible to reduce
use of 'get_node'.
Change 'ipairs' to 'pairs'.
Use 'nodeupdate_single(pos)' instead of 'nodeupdate(pos)' to
avoid every node triggering recursion, the loop itself takes
the place of recursion and works upwards through horizontal
planes as required.

.luacheckrc

@@ -5,7 +5,7 @@ read_globals = {
 	"DIR_DELIM",
 	"minetest", "core",
 	"dump",
-	"vector", "nodeupdate",
+	"vector", "nodeupdate", "nodeupdate_single",
 	"VoxelManip", "VoxelArea",
 	"PseudoRandom", "ItemStack",
 }