refactor: decouple memstore from retriever in cache package

What does this MR do?

Retriever is now part of the cache struct.
Entry now correctly represent a cached entry without being coupled to the retriever.

The Refresh method has also been moved and tests have been updated to use the new method.

Closes #368 (closed)
Closes #553 (closed)

TODO

• Added the Changelog (e.g. Changelog: feature) trailer to the commits that need to be included in the changelog
• Added unit tests (if required)
• Added acceptance tests (if required)
• Documentation (if required)
• Code review guidelines
• Go Style guidelines