Change RBF feerate bump rule to ensure BIP125 relay conformance

[jkczyz]

The 25/24 multiplicative feerate bump for tx_init_rbf can produce increments too small for Bitcoin Core to relay the replacement transaction under BIP125's minimum relay fee policy (default incrementalRelayFee of 0.1 sat/vB since Bitcoin Core v30.0). Add a minimum additive increment of 25 sat/kw alongside the existing multiplicative rule, using whichever produces a higher feerate.

[jkczyz]

Alternatively, we could do the max of 25/24 and +25 sat/kwu?

[t-bast]

Alternatively, we could do the max of 25/24 and +25 sat/kwu?

I like that, it's too cheap to only bump by 0.1 sat/vB if the previous feerate wasn't negligible (e.g. 10 sat/vB). We want those transactions to confirm quickly, otherwise we're not benefiting from the channel capacity changes.

[rustyrussell]

I think we should do max of the two. I missed that 25 perkw is "0.1 sat per vbyte" on my original read.

[jkczyz]

Great, I'll update the PR and clarify. For the math, if you start with a fee rate of 25 sat / kwu, using the minimum RBF needed would at first double (100%) to 50 sat / kwu. Then, for each additional RBF round, the percentage increase would fall until reaching 25/24 (4.167%) at the 24th round, which would be going from 600 to 625 sat / kwu. Then going forward the 25/24 multiplicative dominates leading to a fixed 4.167% increase for each successive round instead of the fixed 25 sat / kwu increase.

This assumes using the minimum needed increase each round. If a larger amount is used, the switchover would happen sooner.

[jkczyz]

@t-bast @rustyrussell I've updated the PR to use the maximum of the previous requirement and the new requirement. PTAL.