Loop out multiple channels and amounts.

I quite often want to loop out an amount spread over certain channels. So 500k sats from each of 4 specific channels, all in one operation, and therefore one on chain fee.

This would mean overriding the amount selection in the outgoing_chan_set which is currently on a lowest fee basis.

I envision a “spread” parameter that would spread “amt” over the outgoing_chan_set.

Either that or individual amounts specified within the outgoing_chan_set, which would be a more complex and probably not backwards compatible change.

“spread” would be lovely 😊

No file or test is named. Start by tracing the outgoing_chan_set amount-selection path and how loop-out parameters are handled. Define the behavior for a spread amount across specified channels, including validation and compatibility with existing requests; done means one operation can distribute the amount across those channels and use one on-chain fee.

Written by the indexing model from the issue text.