Pull request ElementsProject/lightning#4404

`pay` optimizations round 1

6391
6398
6399
6409
6410
6411
6422
6444
6445
6446
6450
6451
6452
6454
6455
6457

Range Diff Matrix


6391..6398
6391..6399	6398..6399
6391..6409	6398..6409	6399..6409
6391..6410	6398..6410	6399..6410	6409..6410
6391..6411	6398..6411	6399..6411	6409..6411	6410..6411
6391..6422	6398..6422	6399..6422	6409..6422	6410..6422	6411..6422
6391..6444	6398..6444	6399..6444	6409..6444	6410..6444	6411..6444	6422..6444
6391..6445	6398..6445	6399..6445	6409..6445	6410..6445	6411..6445	6422..6445	6444..6445
6391..6446	6398..6446	6399..6446	6409..6446	6410..6446	6411..6446	6422..6446	6444..6446	6445..6446
6391..6450	6398..6450	6399..6450	6409..6450	6410..6450	6411..6450	6422..6450	6444..6450	6445..6450	6446..6450
6391..6451	6398..6451	6399..6451	6409..6451	6410..6451	6411..6451	6422..6451	6444..6451	6445..6451	6446..6451	6450..6451
6391..6452	6398..6452	6399..6452	6409..6452	6410..6452	6411..6452	6422..6452	6444..6452	6445..6452	6446..6452	6450..6452	6451..6452
6391..6454	6398..6454	6399..6454	6409..6454	6410..6454	6411..6454	6422..6454	6444..6454	6445..6454	6446..6454	6450..6454	6451..6454	6452..6454
6391..6455	6398..6455	6399..6455	6409..6455	6410..6455	6411..6455	6422..6455	6444..6455	6445..6455	6446..6455	6450..6455	6451..6455	6452..6455	6454..6455
6391..6457	6398..6457	6399..6457	6409..6457	6410..6457	6411..6457	6422..6457	6444..6457	6445..6457	6446..6457	6450..6457	6451..6457	6452..6457	6454..6457	6455..6457