HTTP caching: fix TODOs and take another look at code

[gterzian]

As part of #24203 I realized once again there are:

1. quite a lot of TODOs,
2. quite a lot of missing pieces not marked as TODOs and maybe even bugs that haven't been noticed yet(the spec might have changed here and there as well),

in the "core" caching logic, which mostly consists of checking headers and cache directives and making decisions based on those.

It would be useful to take another good look at https://github.com/servo/servo/blob/master/components/net/http_cache.rs

in the light of the spec at https://tools.ietf.org/html/rfc7234

The tests are at https://github.com/servo/servo/tree/master/tests/wpt/web-platform-tests/fetch/http-cache, we're passing most of it, see https://github.com/servo/servo/tree/master/tests/wpt/metadata/fetch/http-cache, which means the suite probably requires more tests as well.

[pshaughn]

#24909 and #24952 are a couple specific omissions I've noticed in caching (though the latter might not fall under what you're thinking of as "caching logic")