htlcswitch: settle invoices behind switch [WIP]

[joostjager]

This PR is an exploration of what would happen when we move invoice settling behind the switch instead of inside the channel link.

Goals:

• Leverage CircuitMap to store the link between the final htlc and an invoice (proposal: outgoingchannelid=exithop, outgoinghtlcidx=invoice addindex). This link can be used when hold invoices are settled/failed over rpc.
• Prepare for amp. With amp, invoices are settled via multiple channel links. Therefore the invoice settling needs to be lifted out of the link level.

Implementation:

• Remove the complete 'if exithop' case from the link
• Have the switch match exithop with the outgoing channel id and forward to a new struct InvoiceSettler.
• InvoiceSettler is managing the interaction with the invoice database.

[joostjager]

Main problem atm is when and how to get the invoice addindex as outgoing htlc index in the circuitmap.

[joostjager]

This PR is partially woven into #2022, closing.