Stacked mobs should take shared damage

Right now, when mobs stack up (i.e in a spawner farm), only the top mob takes damage. So a stack of 50 mobs, requires hitting the stack to kill 50, 49, 48, etc until the stack is gone. This takes an incredibly long time and makes virtually all farms useless except for fall damage farm. Mob stacks should take shared damage. So any stack of the same type of mob should require the time to kill 1 of that mob to kill the whole stack.