Add client-side render for back pressure responses

[kpdecker]

This allows for a server that has detected a CPU overload case to push page rendering out to the client rather than handle locally on the server.

[kpdecker]

Psuedo-example of use of this within our hapi environment. heavyInstance refers to instance of Heavy.

** WARN ** Deep integration, use this code at your own risk.

  const pageBackpressure = heavyInstance.policy({
    maxEventLoopDelay: 1000,
  });

  server.method(
    'nextPage',
    (pagePath, queryParams, url) => {
      // We are sending psuedo-request and response objects to the
      // renders to avoid potential cache errors due to generating
      // a cache object using a private req or response object.
      const mockReq = {
        stub: 'Static stub to avoid caching private data',
        url,
      };
      const mockRes = { stub: 'Static stub to avoid caching side effects' };

      const overload = pageBackpressure.check();
      if (overload) {
        server.log(['warn', 'page', 'overload'], {
          pagePath,
          queryParams,
        });
        return renderBackpressureToHTML(
          mockReq,
          mockRes,
          pagePath,
          queryParams,
          app.renderOpts
        ).then((backpressure) => {
          overload.output.payload = backpressure;
          return Promise.reject(overload);
        });
      }

      // Render and cache
      return renderToHTML(
        mockReq,
        mockRes,
        pagePath,
        queryParams,
        app.renderOpts
      ).catch((err) => {
        // Note that we need to augment the error here rather than
        // try to clone it via other means due to the way that Error
        // objects are setup internally.
        err.pagePath = pagePath;
        err.queryParams = queryParams;
        return Promise.reject(err);
      });
    });

[kpdecker]

This exact API did not solve the problem due to asynchrony between the check and the actual react render operation. Closing this and will revisit within our fork with a potential PR once it's up to stuff with our load pattern, at minimum.