Geo - Gitlab::Geo should not serialize ActiveRecord objects

What does this MR do?

We should avoid putting binary ActiveRecord objects in the cache. This MR introduces a new cache Gitlab::JsonCache that writes a string containing a JSON representation of the objects to the cache, and parses the cached value back when requested.

What are the relevant issue numbers?

Closes #8827 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer