Optimize fee probe re-runs

[vindard]

Idea

(see: https://github.com/GaloyMoney/galoy/issues/1306#issuecomment-1134769222)

If a payment flow for a higher amount already exists for the same invoice, explore how we can skip re-running the probe from lnd. Either:

• can we re-use the cachedRoute as is for a lower amount?
• can we reliably edit the cachedRoute in-place for the lower amount and update the payment flow object