pay: fix infinite loop when CLTV budget exceeded

[wqxoxo]

Fixes #8167

[cdecker]

Thanks for the attempt to fix this, however, I do not believe this is correct: generally speaking we split in two cases a) when we attempted this size for 10 times, and b) when we cannot find a route because the part is larger than the largest usable route.

Your fix removes the splitting on route-finding failure, which means a payment that needs to be split will fail, even if it could work perfectly fine. Take the following network for example:

A -- 1 --> B
|          |
1          99
|          |
v          v
C -- 9 --> D

For the payment A -> D for an amount of 1.1, we do not find a route, and your fix aborts right away. But there is a working path, if we split once: p1=0.55 going over A->B->D and p2=0.55 A->C->D, however having aborted earlier your change misses this opportunity. We can discuss whether an earlier abort is useful (I'd say yes), but the abort criteria you chose are too aggressive.