feat(datastore): close connections of retired replicas during DLB

What does this MR do?

Related to DLB: Fix replica connection handling (#1338 - closed). Ensures that connections of retired replicas are properly closed before these are removed from the pool.

Note that the gracefully expiration of removed replicas is out-of-scope of Phase 1. That's part of Phase 2. Until then, when necessary/appropriate, the caller should manually handle potential "database closed" errors when interacting with a replica and fallback to primary.

Author checklist

• Assign one of conventiona-commit prefixes to the MR.
  • fix: Indicates a bug fix, triggers a patch release.
  • feat: Signals the introduction of a new feature, triggers a minor release.
  • perf: Focuses on performance improvements that don't introduce new features or fix bugs, triggers a patch release.
  • docs: Updates or changes to documentation. Does not trigger a release.
  • style: Changes that do not affect the code's functionality. Does not trigger a release.
  • refactor: Modifications to the code that do not fix bugs or add features but improve code structure or readability. Does not trigger a release.
  • test: Changes related to adding or modifying tests. Does not trigger a release.
  • chore: Routine tasks that don't affect the application, such as updating build processes, package manager configs, etc. Does not trigger a release.
  • build: Changes that affect the build system or external dependencies. May trigger a release.
  • ci: Modifications to continuous integration configuration files and scripts. Does not trigger a release.
  • revert: Reverts a previous commit. It could result in a patch, minor, or major release.
• Feature flags
  • This change does not require a feature flag
  • Added feature flag: ( Add the Feature flag tracking issue link here )
• Unit-tests
  • Unit-tests are not required
  • I added unit tests
• Documentation:
  • Documentation is not required
  • I added documentation
• database changes including schema migrations:
  • Change does not introduce database changes
  • MR includes DB chagnes
    • Do not include code that depends on the schema migrations in the same commit. Split the MR into two or more.
    • Manually run up and down migrations in a postgres.ai production database clone and post a screenshot of the result here.
    • If adding new queries, extract a query plan from postgres.ai and post the link here. If changing existing queries, also extract a query plan for the current version for comparison.
      • I do not have access to postgres.ai and have made a comment on this MR asking for these to be run on my behalf.
• Ensured this change is safe to deploy to individual stages in the same environment (cny -> prod). State-related changes can be troublesome due to having parts of the fleet processing (possibly related) requests in different ways.
• If the change contains a breaking change, apply the breaking change label.
• If the change is considered high risk, apply the label high-risk-change
• Changes cannot be rolled back
  • Change can be safelly rolled back
  • Change can't be safelly rolled back
    • Apply the label cannot-rollback.
    • Add a section to the MR description that includes the following details:
      • The reasoning behind why a release containing the presented MR can not be rolled back (e.g. schema migrations or changes to the FS structure)
      • Detailed steps to revert/disable a feature introduced by the same change where a migration cannot be rolled back. (note: ideally MRs containing schema migrations should not contain feature changes.)
      • Ensure this MR does not add code that depends on these changes that cannot be rolled back.

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Reviewer checklist

• Ensure the commit and MR tittle are still accurate.
• If the change contains a breaking change, verify the breaking change label.
• If the change is considered high risk, verify the label high-risk-change
• Identify if the change can be rolled back safely. (note: all other reasons for not being able to rollback will be sufficiently captured by major version changes).