Ehancement #2365 : Add markup root directly in ReactMount cache

@syranide Could you check this ?
I wanted to add checking on node parameter because we need a HTML element, but I'm not sure if this is the best way of doing this.
Also, if this okay, I might write some test for ReactMount.cacheNodeById.
@ThomasCrvsr `node` is a DOM node. `ReactMount.getID(node)` reads the ID from the node and if it isn't in the cache, puts it in the cache, we don't need the return value. So it fulfills all the criteria.