lnd: introduce the ChannelNotifier.

This commit introduces the channel notifier which is a central source
of active, inactive, and closed channel events.

This notifier was originally intended to be used by the `SubscribeChannels`
streaming RPC call, but can be used by any subsystem that needs to be
notified on a channel becoming active, inactive or closed.

It may also be extended in the future to support other types of notifications.

server.go

@@ -28,6 +28,7 @@ import (
 	"github.com/lightningnetwork/lnd/autopilot"
 	"github.com/lightningnetwork/lnd/brontide"
 	"github.com/lightningnetwork/lnd/channeldb"
+	"github.com/lightningnetwork/lnd/channelnotifier"
 	"github.com/lightningnetwork/lnd/contractcourt"
 	"github.com/lightningnetwork/lnd/discovery"
 	"github.com/lightningnetwork/lnd/htlcswitch"
@@ -145,6 +146,8 @@ type server struct {
 
 	invoices *invoices.InvoiceRegistry
 
+	channelNotifier *channelnotifier.ChannelNotifier
+
 	witnessBeacon contractcourt.WitnessBeacon
 
 	breachArbiter *breachArbiter
@@ -274,6 +277,8 @@ func newServer(listenAddrs []net.Addr, chanDB *channeldb.DB, cc *chainControl,
 
 		invoices: invoices.NewRegistry(chanDB, activeNetParams.Params),
 
+		channelNotifier: channelnotifier.New(chanDB),
+
 		identityPriv: privKey,
 		nodeSigner:   netann.NewNodeSigner(privKey),
 
@@ -986,6 +991,9 @@ func (s *server) Start() error {
 	if err := s.cc.chainNotifier.Start(); err != nil {
 		return err
 	}
+	if err := s.channelNotifier.Start(); err != nil {
+		return err
+	}
 	if err := s.sphinx.Start(); err != nil {
 		return err
 	}
@@ -1083,6 +1091,7 @@ func (s *server) Stop() error {
 	s.authGossiper.Stop()
 	s.chainArb.Stop()
 	s.sweeper.Stop()
+	s.channelNotifier.Stop()
 	s.cc.wallet.Shutdown()
 	s.cc.chainView.Stop()
 	s.connMgr.Stop()