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.
`ReactDOMIDOperations.dangerouslyReplaceNodeWithMarkupByID` is called by `ReactCompositeComponent.updateComponent` which takes care of the mounting and unmounting. The unmounting of the old component clears its node from the cache.