lnd: introduce the utxoNursery which incubates outputs until maturity

This commit introduces the utxoNursery. The duty of the utxoNursery is
to watch over CSV-locked immature outputs until they’ve fully matured.
An output is mature once both its sequence lock indicated by the CSV op
code within its output has become active. Once an output is mature the
nursery sweeps the outputs in batches into the source wallet.

The utxoNursery executes its duties once a commitment transaction has
been broadcast on-chain.

server.go

@@ -55,6 +55,8 @@ type server struct {
 
 	routingMgr *routing.RoutingManager
 
+	utxoNursery *utxoNursery
+
 	newPeers  chan *peer
 	donePeers chan *peer
 	queries   chan interface{}
@@ -104,6 +106,8 @@ func newServer(listenAddrs []string, notifier chainntnfs.ChainNotifier,
 	// TODO(roasbeef): remove
 	s.invoices.addInvoice(1000*1e8, *debugPre)
 
+	s.utxoNursery = newUtxoNursery(notifier, wallet)
+
 	// Create a new routing manager with ourself as the sole node within
 	// the graph.
 	s.routingMgr = routing.NewRoutingManager(graph.NewID(s.lightningID), nil)
@@ -145,6 +149,9 @@ func (s *server) Start() error {
 	if err := s.htlcSwitch.Start(); err != nil {
 		return err
 	}
+	if err := s.utxoNursery.Start(); err != nil {
+		return err
+	}
 	s.routingMgr.Start()
 
 	s.wg.Add(1)
@@ -175,6 +182,7 @@ func (s *server) Stop() error {
 	s.fundingMgr.Stop()
 	s.routingMgr.Stop()
 	s.htlcSwitch.Stop()
+	s.utxoNursery.Stop()
 
 	s.lnwallet.Shutdown()