Refactoring NodeChild and Node.update.

Plus removing of some maps in children update-ing, which boost performance. 

Now, on my computer it is approximately similar in simple speed-test.

I didn't inline `_updateChildren` method, because there is some more complicated algorithm and i wanted it in separate file from basic `Node` structure. There is no performance difference, when it is inined, but if there is some reason to inline it, it is no problem.