Make service worker fetch updates in parallel

[RichDom2185]

Following code-splitting, we now have a lot of independent bundles. However, the service worker fetches all of them only sequentially:

This results in the exceedingly long time before any update is fully downloaded (and the app is ready to update). It would be great if the service worker can fetch the assets in parallel.

[RichDom2185]

@chownces do you know if this is possible?