Performance issues of scrollBehavior

[fnlctrl]

Currently, using scrollBehavior adds a scroll listener to record window.pageXOffset and window.pageYOffset, which will force the browser into doing layout+repaint when scrolling, resulting in serious scrolling janks on mobile devices.

Since we cannot avoid accessing pageXOffset and pageYOffset, here are some improvements I can think of:

1. Implement a simple debounce function for the scroll listener, and add a scrollListenerDebounce option to router configs, which defaults to 1000ms. This should make accessing pageXOffset and pageYOffset only when the user have stopped scrolling for 1000ms.
2. Add a section in the docs explaining the potential performance issues when using scrollBehavior, and the inaccuracy of the recorded scrolling position, caused by the debounce function