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.
I know pointed you in this direction, but it should actually be preferable to just use `ReactMount.getID` instead. PS. Also you can't use the `id` argument because it's for the old node, not the new.