Refactor

[efstajas]

This service is a bit of a mess and needs a proper overhaul.

1. The readability of the core fallback logic is terrible. It should be broken into modules and completely refactored while preserving the same behavior.

2. Despite the very simple route structure, we should use the oak framework for routing, rate limiting middleware, and request processing.

3. Add tests for the following scenarios:

• First RPC successful (normal)
• First RPC fails, second successful (returns 2nd res)
• First RPC hangs, second successful (returns 2nd res)
• All RPCs fail (returns status and body of last request)