[feature]: granular decrease in added feerate security buffer

[JssDWt]

Is your feature request related to a problem? Please describe.
In #8096 a fee buffer was added for routing htlcs. The buffer contains 2x the current chain fee rate + 1 additional htlc. In very high-fee scenarios the 2x fee buffer causes small channels to be unable to forward any htlcs. In very high fee scenarios adding 2x the chain fee rate is a much larger increase than in low-fee scenarios, effectively jamming channels entirely.

Describe the solution you'd like
Add a granular decrease in the added buffer based on the current chain fee estimate. CLN has implemented this granular decrease already in ElementsProject/lightning#7063, with accompanying issue ElementsProject/lightning#6974. See theory there.

Here's the expected curve for the added buffer fee rate.

[ziggie1984]

I think that makes sense thank you for the idea!

As long as the the fee is calculated percentage wise, you can limit the feeBuffer by limiting the ; max-commit-fee-rate-anchors setting.