Provide callbacks in serviceWorker (next)

[piotr-cz]

This is PR version of #3375 for next branch.

Adding two callbacks in serviceWorker.js file:

• onUpdate - executed when service worker has been updated in the background
  (console logs New content is available; please refresh.)
• onSuccess - executed after service worker has been installed for the first time
  (console logs Content is cached for offline use.)

To use it inside application to show notification (toast/ icon etc) register service worker inside App.js file:

// App.js

import { register as registerServiceWorker } from './serviceWorker.js'

export default class App {
  constructor() {
    // Register service worker and add onUpdate callback
    registerServiceWorker({
      onUpdate: this.handleServiceWorkerUpdate
    })
  }

  // Simplest notification implementation example.
  handleServiceWorkerUpdate(registration) {
    if (window.confirm('New update has been installed, click to restart')) {
      window.location.reload()
    }
  }
}

[gaearon]

This looks good, thank you!