lnd: add support for multi-hop (Sphinx) onion routed payments

This commit adds full support for multi-hop onion routed payments
within the daemon.

The switch has been greatly extended in order to gain the functionality
required to manage Sphinx payment circuits amongst active links. A
payment circuit is initiated when a link sends an HTLC add to the
downstream htlcSwitch it received from the upstream peer. The switch
then examines the parsed sphinx packet to set up the clear/settle ends
of the circuit. Created circuits can be re-used amongst HTLC payments
which share the same RHash.

All bandwidth updates within a link’s internal state are now managed
with atomic increments/decrements in order to avoid race conditions
amongst the two goroutines the switch currently uses.

Each channel’s htlcManager has also been extended to parse out the
next-hop contained within Sphinx packets, and construct a proper
htlcPkt such that the htlcSwitch can initiate then manage the payment
circuit.

invoiceregistry.go

@@ -110,7 +110,7 @@ func (i *invoiceRegistry) LookupInvoice(rHash wire.ShaHash) (*channeldb.Invoice,
 // dbueg invoice, then this method is a nooop as debug invoices are never fully
 // settled.
 func (i *invoiceRegistry) SettleInvoice(rHash wire.ShaHash) error {
-	ltndLog.Debugf("Setting invoice %x", rHash[:])
+	ltndLog.Debugf("Settling invoice %x", rHash[:])
 
 	// First check the in-memory debug invoice index to see if this is an
 	// existing invoice added for debugging.