Fix held attack key not hitting mobs bug (this time for sure)

Reverting to old code in the previous PR but with the new optimizations kept

mods/ENTITIES/mcl_mobs/combat.lua

@@ -532,11 +532,10 @@ function mob_class:on_punch(hitter, tflp, tool_capabilities, dir)
 		return
 	end
 
-	-- invulnerability time
-	local invul_time = 0.4
+	local time_now = minetest.get_us_time()
 
-	-- check for invulnerability
-	if tflp <= invul_time then
+	-- check for invulnerability time in microseconds (0.5 second)
+	if time_now - self.last_invul_time <= 500000 then
 		return
 	end
 
@@ -665,6 +664,9 @@ function mob_class:on_punch(hitter, tflp, tool_capabilities, dir)
 			-- do damage
 			self.health = self.health - damage
 
+			-- give invulnerability
+			self.last_invul_time = time_now
+
 			-- skip future functions if dead, except alerting others
 			if self:check_for_death( "hit", {type = "punch", puncher = hitter}) then
 				die = true