approach-3: Set outgoing accountable in process_pending_forwards

[carlaKC]

In this approach, we just copy the incoming signal in process_pending_forwards, rather than setting it in forward_htlcs. This has the advantage of not needing to store our outgoing_accountable signal anywhere, as we have the incoming_accountable signal on hand.

I like the flexibility of this approach in particular, because we have some flexibility:

• If we put interception in forward_htlcs, we can just add it to PendingHtlcRouting
• If we put interception in process_pending_forwards, then we're good to go