Improve async handling

Hi
It would be good to have an interface like this:

render(
  <Provider store={store}>
    <Root/>
  </Provider>
).then(({html, context, state}) => { /* all done, serve to client now */ }

So the render promise will only resolve once everything has been rendered and all async actions have been finished. This would make it easier to get the redux state too and you wouldn't have to get the redux state artificially before calling render (plus artificially figuring out the route n stuff - all the things that make the server side different from the client side).
Basically run the complete application server side until its stabilized and ready to render.

Start at the render entry point and trace how server-side rendering currently handles asynchronous actions and Redux state. Define the stabilization boundary and the returned html, context, and state values described in the issue; done means the render promise resolves only after the complete application is ready to serve.