Fix force_shutdown() bug where we lose knowledge of a preimage

Fix force_shutdown() bug where we lose knowledge of a preimage

In case we are in AwaitingRemoteRevoke and we go to claim an HTLC
(at which point we've already given up the money to the next hop in
the payment) we just write it to holding_cell_htlc_updates.
However, we should be ensuring we *also* write it to our
channel_monitor as we need to make sure we can still claim it after
a force_shutdown() or otherwise after hitting the chain.