Geo: read-only safeguards was not working on Secondary node

What does this MR do?

Fixes Gitlab::Database.read_only? in EE. Old code was trying to use extend to patch existing code, but the way it was done was not working as expected, see explanation here: https://gitlab.com/gitlab-org/gitlab-ee/issues/3876#note_45127903

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Regression introduced in %10.1 breaking read-only safeguards for Geo secondary node.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #3876 (closed)

cc @stanhu @to1ne @nick.thomas