askrene: detect and cancel flow cycles

[Lagrang3]

Flow cycles can occur if we have zero arc costs.
The previous path construction from the flow in the network assumed the absence of
such cycles and would enter an infinite loop if it hit one.

With his patch we add cycle detection and removal during the path construction phase.

Reported-by: Rusty Russell rusty@rustcorp.com.au

[Lagrang3]

I think this is roughly what we need: detect cycles and remove them.
Another option is to ensure that there are no zero cost arcs ... I am not sure that would be the right choice.

I leave it as a draft today because it fails the tests. Clearly I made a mistake somewhere.
The logic branch when we match a source and a sink node is pretty much the same code we had before.
However, the function that erases cycles is not yet tested.