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.
One last thing :)
A comment briefly explaining that `getID` puts the node in the cache and that we do this to avoid holes in the cache (which can otherwise cause all siblings to be unnecessarily re-cached). It would be great info for anyone else stumbling upon this code.