HTTP cache internals and monitoring routes: small cleanups

[allen0099]

HTTP cache internals and monitoring routes: small cleanups. Not breaking.

Split into one sub-issue per pull request:

• @cache wrapper: build the key lazily and tidy the request check #182
• CacheManager._encode encodes the JSON string twice #183
• Monitoring routes: report media_type and rename the key-split constants #184

[allen0099]

All sub-issues are done; closing.