- create the caches interface and matching cachesImpl
- move the cache logic out of handler
- openDB
- readCache
- useCache
- gcCache
- access to the storage struct
- serve
- commit
- exist
- write
- add getCaches / setCaches to the handler interface so it can be
used by tests. The caches test should be implemented independently
in the future but this is a different kind of cleanup.
- no functional change, minimal refactor
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