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.
Grammar is a little off, something like this perhaps:
```
// `getNode` populates ReactMount's node cache with all siblings, but the
// replaced node creates a hole. `getID` fills the hole with the new node.
```