Spec network revocation mechanism to disable fetches

[gtanzer]

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[domfarolino]

Should this PR be marked as a draft for now? Or is it ready for review? It does not look ready, but I'm just double checking.

[gtanzer]

@domfarolino Is there a way to explicitly mark it draft? I didn't request a review. I'll just add WIP to the title.

[gtanzer]

@domfarolino I think the content is "done" now, % resolving some links. But it would be good if you could take a look at it first so I don't resolve links that will be deleted/moved around later.

The part I'm least sure about is where to define the extra objects on the user agent. In the fetch spec, it defines the connection pool on the "user agent" which isn't a formal object, so I don't know if I should monkeypatch these definitions near there or just keep them in here.