I was unable to do a "Max" amount submarine swap because the
`fee_estimate` method 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 determined 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 the balance of
the channel.
Using the fees the maximum fees user has configured and estimate the
potential fee as inverse of PaymentFeeBudget is more
reliable/conservative as we definitely aren't going to pay more fees
than this amount.
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