Generate a PaymentForwarded event when a forwarded HTLC is claimed

It is useful for accounting and informational reasons for users to
be informed when a payment has been successfully forwarded. Thus,
when an HTLC which represents a forwarded leg is claimed, we
generate a new `PaymentForwarded` event.

This requires some additional plumbing to return HTLC values from
`OnchainEvent`s. Further, when we have to go on-chain to claim the
inbound side of the payment, we do not inform the user of the fee
reward, as we cannot calculate it until we see what is confirmed
on-chain.

Substantial code structure rewrites by:
Valentine Wallace <vwallace@protonmail.com>