fix: don't use handle_update_instance_request_and_wait in governor

Summary

• fix: don't use handle_update_instance_request_and_wait in governor
• refactor: extract prepare_update_instance_cas_request function for reusability
• refactor: don't use storage in update_instance
• chore: add a debug assertion to check that connection pool works correctly
• fix: remove redundant timeouts on futures
• fix: cleanup timeouts a bit in governor
• fix: don't forget the error code

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• Cherry-pick to: none
• Docs follow-up: not necessary