[ServiceWorker] Changing Request and fetch() (#625)

We want to expose these features over time and the best candidate is the `Request` object:

1) Messaging with the service worker's `FetchEvent`.
2) Aborting a fetch.
3) Modifying a fetch on the go (e.g. changing priority).

I think this requires that we change `Request` that after passing it to `fetch()` it is no longer usable. And change `fetch()` to not copy the object, but instead mutate it.

#624 is related to this.

Please let me know what you think about this soonish so I start making the required changes.

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Reply to this email directly or view it on GitHub:
https://github.com/slightlyoff/ServiceWorker/issues/625