feat(cache)!: tag HTTP cache keys http:v2 and separate them with one |

[allen0099]

Second of the 0.4.0 cache-key changes, on top of CacheKey (#405).

What changes

• CACHE_KEY_SEPARATOR is "|". Method, host, path and extra components are percent-encoded and the query is URL-encoded, so none of them can contain it (0.4.0: use a single | as the cache-key separator #271).
• Every HTTP key starts with the format tag http:v2 (CacheKey.FORMAT_TAG): http:v2|method|host|path|query|extra.... A later format gets another tag, and clear_pattern("http:v2|*") removes every key of this one (Cache key: add a format version tag to HTTP keys #266).
• CacheKey.parse() returns None for untagged keys. clear_path() and the monitoring routes therefore skip 0.3.x keys and keys from a key_builder that builds its own string instead of calling build_cache_key(). Such builders still cache, invalidate() and clear_pattern(); the docs say so.
• CacheKey.path_glob() includes the tag.
• The path-shaped clear_pattern warning now suggests http:v2|GET|*|<pattern>|*, which also matches a path with no glob.

Docs

• CACHE_FLOW.md, HTTP_CACHING.md and MIGRATING_0_4.md, in English and zh-TW:
  • new format and examples;
  • the untagged custom key note;
  • "exact tag not final" removed;
  • the Redis clear_pattern example is now "http:v2|GET|*";
  • a caution that the *|||* cleanup pattern matches any key containing |||.
• changelog.d/125.removed.md now gives the 0.4.0 pattern.

Tests

• Key literals across the suite are moved to the new format.
• New cases for parse() rejecting 0.3.x, other-tag, untagged, query-less and method-less keys.
• path_glob matches only tagged keys.
• The pattern the warning suggests actually clears the entries.
• clear_pattern("*|||*") removes 0.3.x keys and leaves current ones.

Closes #271
Closes #266