separate objects in js cache according to size
Big objects in js cache slow down browser.
Use at least 3 levels of js cache:
- num media > 1000: retain only 1 object (and compress it?)
- num media > 200: retain 5 objects (and compress it?)
- others: retain all objects
Adopt a new option:
js_cache_levels = [(1000, 1), (200, 5)]