Continue http cache work
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Work in progress, and not quite worth a review yet. (Continuation of https://github.com/servo/servo/pull/4117)
TODO
- [ ] cache metadata (find some subset of`net_traits::Metadata` that can be shared across threads, it seems the problem is mainly stuff inside `hyper::header` in the `headers` field)
- [ ] determine which other fields of a `Response` need to be cached, so a full and valid one can be returned upon a cache hit.
- [ ] determine how to best share the cache across fetch threads (inside HttpState like I tried now?)
- [ ] Spend more time reading the spec and make sure the cache follows it where it matters.
- [ ] Make the current wpt tests pass.
- [ ] More...
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- [ ] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy` does not report any errors
- [ ] These changes fix#12972 (github issue number if applicable).
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