fix serious performance problem of Animate.

this is a serious bug, it cause massive cpu consume.
if we have 1000 animate in active scene, this pr can raise 5% performance globally, raise about 70% performance locally of Animate!!!!!!!!!!!! .
because it have massive calls cocos2d::Vector constructor and de-constructor. 
Vector's constructor will also call "addRefForAllObjects"(for loop)!!!!!


This problem is very, very serious,  and still has many similar errors in cocos2d-x.

we must use "auto&" sometimes, especially in update per frame. 
Thanks for your detail explain.
I agree with you we should use insert instead of operator[].

> Many programmers don't know that operator[] will create a new element when it does not exist.

This is true for map but not true for vector. Right?