When we get a fail/fulfill on an outgoing HTLC, we tell the correspoding
incoming HTLC about it. But if that peer is disconnected, we don't.
The better solution is to copy the preimage/malformed/failmessage and mark
the incoming HTLC as resolved. This is done most simply by marking it
SENT_REMOVE_HTLC, which will work in the database case as well.
channeld now re-transmits appropriately when it gets started with an HTLC
in that state.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>