- previous pending sendpays must add up so that the plugin tries to pay
the rest of the amount,
- avoid groupid, partid collisions,
- add shadow fees if the option is set and the payment amount - total
  delivering = 0
- add a test,
- also fix a buggy shadow routing test

Whenever there is a payment failure that requires gossip update, for
example changing the fee rates of remote channels, we call addgossip.
For renepay to consider this changes in the coming payment attempts, it
must update gossmap.