xpay: 'punish' node pairs rather than individual channels

xpay appears to retry the same node pairs often.

In below example 5 out of 6 attempts try the connection between Kraken and e960fd8385a1603003727. However, if one of the channels between 2 nodes doesn't work, you can assume the other channels were also not able to carry the htlc. At least core lightning itself, and also LND, will forward a htlc over a channel of _their_ choosing, rather than the sender picked channel. Therefore, if one of the channels can't route the htlc, you can assume any of the channels between these two peers can't route the htlc. A good measure I think is to 'punish' each channel between the two peers, rather than just the one.

```
Error: Error code 209: Timed out after after 6 attempts. We got temporary_channel_failure for 843729x1660x0/0, assuming it can't carry 1300650msat. We got temporary_channel_failure for 893749x3026x0/0, assuming it can't carry 1302298msat. We got temporary_channel_failure for 893749x2860x1/1, assuming it can't carry 1302298msat. We got temporary_channel_failure for 841972x3032x0/0, assuming it can't carry 1300650msat. We got temporary_channel_failure for 845984x584x0/0, assuming it can't carry 1300650msat. We got temporary_channel_failure for 829718x2353x0/1, assuming it can't carry 1300650msat.
```