Implement server-side rendering support

[kitten]

Fix #218

Server-side rendering can now be achieved; an app using urql can do this with just minor modifications. Instead of using getDataFromTree like react-apollo does we're using our more generic react-ssr-prepass library.

This still needs to be documented but essentially on the server the client used like so:

import ssrPrepass from 'react-ssr-prepass';

import {
  createClient,
  dedupExchange,
  cacheExchange,
  ssrExchange,
  fetchExchange
} from 'urql';

// We create an ssrExchange, which is an additional cache for client-side rehydration
const ssr = ssrExchange();

// The client can also be shared, but `suspense` needs to be set to
// false on the client-side
const client = createClient({
  url,
  exchanges: [dedupExchange, cacheExchange, ssr, fetchExchange],
  suspense: true
});

// The user has their app somewhere
const App = () => {/* ... */};
// We execute the SSR prepass
await ssrPrepass(<App />);

// Now we can extract the data:
const data = ssr.extractData();

// ...
// This is where renderToString or something goes
// We then need to send down the urql ssr data in the HTML as well
const html = `<script>window.DATA = (${JSON.stringify(data)});</script>`

And on the client we also use the SSR exchange to rehydrate the data:

import {
  createClient,
  dedupExchange,
  cacheExchange,
  ssrExchange,
  fetchExchange
} from 'urql';

// We create an ssrExchange, which is an additional cache for client-side rehydration
const ssr = ssrExchange({ initialState: window.DATA });

// The above can also be used as:
// ssr.restoreData(window.DATA);

const client = createClient({
  url,
  exchanges: [dedupExchange, cacheExchange, ssr, fetchExchange],
  suspense: false // NOTE this is false on the client
});

// ...
// ReactDOM.rehydrate or otherwise

[kitten]

I'm currently investigating a remaining issue where during client-side rehydration a race condition leads to a request being sent unnecessarily due to a race condition (potentially?)

Edit: nevermind, it's fixed

[kitten]

Will potentially be retroactively reviewed again

[helloguille]

Works nicely. I'll keep testing