fix(cache)!: store and replay repeated response headers line by line

[allen0099]

Closes #105, following the revised design in the issue.

What changes

• CacheEntry.headers type. It is now a tuple of (name, value) pairs in the order sent, instead of dict[str, str] | None; no headers is ().
  • The constructor still accepts a dict, any iterable of pairs, or None, and converts it.
  • It raises TypeError for anything but (str, str) pairs.
• What @cache stores and replays. It stores every header line. A hit and all three 304 paths (stored entry, no_cache, bypass/private) replay them in order.
  • Several Link lines, or a custom header read with getlist(), come back as sent.
  • Values are not joined, so a value holding a comma stays one line.
• Headers a hit sets itself. It filters the stored lines through the uncacheable set, then sets its own Content-Type, Content-Length, ETag, Cache-Control and Age. An entry built outside @cache can therefore not duplicate framing headers or override the validators.
• Codec (Redis, Memcached). It writes [[name, value], ...] and still decodes the 0.3.x object. Any other shape is a miss.
• add_vary(). It puts the missing names on a new Vary line. Starlette's add_vary_header rewrote the first line and dropped the others. As a result, @cache(vary=...) and the session middleware lost a handler's second Vary line; that was already the case on master.

Migration

MIGRATING_0_4.md#cache-entry-headers now says which setups are affected in a rolling deploy.

• With the default key builder, or one that calls build_cache_key(), a rolling deploy needs nothing: the key format changed in the same release, so 0.3.x and 0.4.0 never read each other's HTTP entries.
• A custom key_builder that builds the key itself gives the same key in both versions. 0.3.9 answers 500 on an entry 0.4.0 wrote with headers. For such a route, keep 0.3.x off the shared backend during the rollout, or move the builder to build_cache_key() first.

No version marker is needed for this.

Docs

• CACHE_FLOW, HTTP_CACHING and MIGRATING_0_4 are updated in English and zh-TW.
• Changelog: changelog.d/105.changed.md.

Tests

• New tests/test_cache_repeated_headers.py covers:
  • storing and replaying every line in order;
  • 304 Vary lines on all three paths, with and without vary=;
  • a hit from an entry carrying framing and validator headers;
  • codec round-trip, the 0.3.x object, and malformed shapes treated as a miss;
  • CacheEntry conversion and rejection;
  • add_vary when every name is already present.
• Existing tests are updated from headers is None to (). The codec round-trips now use repeated link lines.
• Checked that the tests catch each of these mutations:
  • collapsing to a dict on store or on the 304;
  • reverting add_vary;
  • dropping the replay filter;
  • loosening or dropping the pair and type checks;
  • accepting only a dict;
  • appending an empty Vary line.