Implement Cache​Storage

[davidbarratt]

I noticed that CacheStorage hasn't been implemented yet, so here's a task to implement it. Thank you for all your hard work! :)

[zekth]

There is already an issue about the implementation of different kind of Storage : #1657

[davidbarratt]

There is already an issue about the implementation of different kind of Storage : #1657

That's a different API / Interface. :)

[zekth]

If you read the whole issue there is same concern about Cache Session and so on

[davidbarratt]

If you read the whole issue there is same concern about Cache Session and so on

Ah!

[kitsonk]

I know this came well before #5957 but we should consolidate everything around implementing Service Workers as a whole, not just parts of the API. So closing this as a duplicate of #5957.