Almost complete rewrite of cache filter

[ravenblackx]

Commit Message: Almost complete rewrite of cache filter
Additional Description: WIP - does not yet support vary headers (turns out the old one only did it poorly too, but I guess we can't step one thing backwards to fix all the other things this fixes). This is necessarily a massive change because the old cache filter was almost non-functional. Important differences:

1. it's now resilient against "thundering herd" problem - only one request goes upstream if there are multiple simultaneous requests for the same resource.
2. it now works with range requests, which, if there previously were only range requests, as can happen with Microsoft BITS as a client, would all just bypass the cache. The new behavior is a range request causes a full file request upstream, and then serves the range response once the requested part of the file is cached. This could be improved by also supporting partial cache entries, but it's reasonable for that to be a later PR.
3. a lot of work that the cache filter previously offloaded to the cache implementation is now owned by the cache filter (e.g. vary headers, validation, thundering herd resistance). This makes cache implementation much less onerous, but adds some intermediate layers.
   Risk Level: Pretty bad, but the filter is still tagged WIP, so not critical.
   Testing: There's quite a lot, but still needs a bit more coverage. Existing integration tests still function mostly as they were, and added integration tests for range and parallel requests which would have failed with the old cache.
   Docs Changes: Internal only.
   Release Notes: Not yet.
   Platform Specific Features: n/a

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🐱

Caused by: #37990 was opened by ravenblackx.

see: more, trace.