config: add a new --debughtlc config parameter

This commit adds a new configuration parameter to the deamon:
‘DebugHTLC’. When true, all outgoing HTLC’s sent via the RPC interface
will be sent paying to a special rHash value which all lnd nodes also
with the flag activated know the preimage to. Therefore all payments
sent to a 1-hop node will immediately be settled by that node.

By default, this flag is false, it it only intended to be used to
exercise local changes to 1-hop behavior manually.

server.go

@@ -104,6 +104,15 @@ func newServer(listenAddrs []string, notifier chainntnfs.ChainNotifier,
 		quit:          make(chan struct{}),
 	}
 
+	// If the debug HTLC flag is on, then we invoice a "master debug"
+	// invoice which all outgoing payments will be sent and all incoming
+	// HTLC's with the debug R-Hash immediately settled.
+	if cfg.DebugHTLC {
+		kiloCoin := btcutil.Amount(btcutil.SatoshiPerBitcoin * 1000)
+		s.invoices.AddDebugInvoice(kiloCoin, *debugPre)
+		srvrLog.Debugf("Debug HTLC invoice inserted, preimage=%x, hash=%x",
+			debugPre[:], debugHash[:])
+	}
 
 	s.utxoNursery = newUtxoNursery(notifier, wallet)