Dynamic polling interval

[shuding]

When changing refreshInterval dynamically:

const [int, setInt] = useState(0)
const { data } = useSWR('/api', { refreshInterval: int })

return <>
  {data}
  <button onClick={() => setInt(int + 1000)}>update</button>
</>

Sometimes the received data will not be updated from the data source upon interval changes. Need to refine the interval change logic and remove refreshInterval from the useEffect deps, and use a ref for it.

Also we need a test to make sure it'll be stable in the future.