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.
It's removed by whoever calls `ReactDOMIDOperations.dangerouslyReplaceNodeWithMarkupByID`. Which is another bad thing, we effectively force a re-cache of the parent of every node we update, even if they are already cached.