Add `maxfeepercent` for C-Lightning `pay` command to permit higher payment success rate

[willcl-ark]

I have been having some difficulty paying to Muun wallet from my C-Lightning node, as they appear to have some very high-fee channels on the recipient side at the moment...

This is solvable when interacting directly with my node, by increasing the allowed maxfeepercent parameter on the pay command, e.g. instead of:

lightning-cli pay bolt11

...which will fail with "fee exceeds our fee budget", I can use:

lightning-cli pay -k bolt11=invoice maxfeepercent=0.6

...which increases the fee budget and can therefore find more routes to try.

I think it would be generally useful if we could somehow integrate a maxfeepercent toggle into the UI? Perhaps it could appear only after first payment failure, if it's deemed "too advanced" (or to be even more precise, could appear if the returned failreason contains Fee exceeds our fee budget, or else could be in an "advanced" tab when scanning the invoice?

[kaloudis]

@Bosch-0 let's add this to the v0.6.1 sprint. Definitely need this.

[deregs]

#860

[Bosch-0]

Sounds good to me! Looks like it can be apart of 0.6.0 thanks to Deregs <3

[kaloudis]

This has been added to v0.6.0. Can you test it out @willcl-ark?

[willcl-ark]

Thanks guys. I have tested this on 0.6 today but unfortunately fee situation has improved on Muun wallet node, so I wasn't able to invoke the new behaviour even with a 5 sat payment :(

After the conference I plan to create a dummy node which will have a very high fee channel so that I can force the new mode.

[willcl-ark]

I think I can close this for you now, right? let me know if you want it re-opened for some reason.