[whatwg/fetch] Define the "Preload Cache" (#590)

We need to define the preload cache, as currently it is not defined and different implementations are doing [observably different things](https://bugzilla.mozilla.org/show_bug.cgi?id=1222633#c38).

This issue was [opened](https://github.com/w3c/preload/issues/97) on the Preload spec, but as the cache would mostly sit inside Fetch and it's not clear what action would be needed on the preload spec side, I'm "moving" it here.

Related previous discussion here was at https://github.com/whatwg/fetch/issues/354 where @jakearchibald made a [proposal](https://github.com/whatwg/fetch/issues/354#issuecomment-237508188) to address this. I open this as a separate issue as I think the H2 push cache and the preload cache are inherently different things in different layers.

I think that anything we define here should, to some extent, look at how the different implementations tackle this today. The logic that they apply for this (e.g. in [Blink](https://cs.chromium.org/chromium/src/third_party/WebKit/Source/platform/loader/fetch/ResourceFetcher.cpp?sq=package:chromium&l=997) or in [WebKit](https://github.com/WebKit/webkit/blob/master/Source/WebCore/loader/cache/CachedResourceLoader.cpp#L946)) seems a bit complex, but was created for the "memory cache" case. We need to think if it can be safely simplified for the preload case. (and if memory cache itself should be standardized)



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