lnworker: use config lightning fee for estimate

[f321x]

I was unable to do a "Max" amount submarine swap because the fee_estimate function used by LNWallet.num_sats_can_send() uses a hardcoded fee_proportional_millionths to estimate the fee for the lightning payment.
When the actual fee calculated later is higher than the estimated fee the payment fails as the channel is unable to add the htlc sum including the real fees as the amount exceeds what the channel can add.
Using SimpleConfig.LIGHTNING_PAYMENT_FEE_MAX_MILLIONTHS seems more reliable here as it will always use the possible maximum fee the user is willing to pay for the fee estimate.

[SomberNight]

Using SimpleConfig.LIGHTNING_PAYMENT_FEE_MAX_MILLIONTHS seems more reliable here as it will always use the possible maximum fee the user is willing to pay for the fee estimate.

Well, with your current diff, you are estimating with fee_base_msat = 5000 plus config.LIGHTNING_PAYMENT_FEE_MAX_MILLIONTHS. So for large payments, because of that 5 sat base fee, you are overestimating a bit.
For small payments, because of the cutoff, you are underestimating.

What I mean is that lnworker.fee_estimate is currently inverting:

electrum/electrum/lnrouter.py

Lines 59 to 61 in 5302606

	def fee_for_edge_msat(forwarded_amount_msat: int, fee_base_msat: int, fee_proportional_millionths: int) -> int:
	return fee_base_msat \
	+ (forwarded_amount_msat * fee_proportional_millionths // 1_000_000)

but maybe what you are trying to do - if you just want to assume max fees - is to instead invert:

electrum/electrum/lnutil.py

Lines 2020 to 2023 in 5302606

	# for small payments, fees <= constant cutoff are fine
	# for large payments, the max fee is percentage-based
	fee_msat = invoice_amount_msat * millionths_clamped // 1_000_000
	fee_msat = max(fee_msat, cutoff_clamped)