feat(ServerRendering) support per-request renderers

[rmosolgo]

Currently, a renderer is "checked out" for each call to react_component. However, if you want some state in that context, it's inefficient: you have to load the state before each call, then clear it after each call.

This PR adds support for one-renderer-per-request, so you can load the JS state before rendering the view, then unload it afterwards.

You could do like this:

class ThingsController < ApplicationController 
  # This around-action is provided by react-rails: 
  per_request_react_renderer

  def show 
     # `react_rails_prerenderer` is assigned by react-rails 
     js_context = react_rails_prerenderer.context 
     js_context.eval(setup_code)
     render :show 
     js_context.eval(teardown_code)
  end 
end 

Of course, you could further generalize this solution with before_/after_/around_actions

Todo

• End-to-end test using __prerenderer.context
• Improve the API?