blip-0041: channel funding fee credit

[t-bast]

Onboarding users to the lightning network is hard, because opening a channel requires paying on-chain fees and allocating capital, hoping that it will be used to earn routing fees. It is not economical nor sustainable to open channels for users that receive a tiny amount and may not use the network frequently enough.

Wallet providers usually work around this issue by requiring users to receive a large enough first payment to pay on-chain fees for their channel.

However, a collection of unrelated tiny payments received over a period of time may be enough to justify creating a channel. We thus introduce a funding fee credit, that lets users prepay for future channel(s) by proactively revealing htlc preimages for small payments. When that fee credit reaches a sufficient amount, it is used by the remote node to cover the on-chain costs of opening a channel or splicing additional liquidity.