Claim HTLC output on-chain if preimage is recv'd after force-close

Claim HTLC output on-chain if preimage is recv'd after force-close

If we receive a preimage for an outgoing HTLC that solves an output on a
backwards force-closed channel, we need to claim the output on-chain.

Note that this commit also gets rid of the channel monitor redundantly setting
`self.counterparty_payment_script` in `check_spend_counterparty_transaction`.

Co-authored-by: Antoine Riard <ariard@student.42.fr>
Co-authored-by: Valentine Wallace <vwallace@protonmail.com>