chanfitness: Add channel event store

This commit adds a channel event store to the channel fitness
package which is used to manage tracking of a node's channels.
It adds tracking for channel open/closed and peer online/offline
events for all channels that a node has open.

Events are consumed from channelNotifier and peerNotifier event
subscriptions. If either of these subscriptions is cancelled,
channel scoring stops, because both subscriptions are expected
to run until node shutdown.

Two functions are exposed to allow external callers to get uptime
information about a channel. GetLifespan returns the period over
which the channel has been monitored. GetUptime returns the channel's
uptime over a specified period. Callers can use these functions to
get the channel's remote peer uptime over its entire lifetime, or
a subset of that period.

server.go

@@ -30,6 +30,7 @@ import (
 	"github.com/lightningnetwork/lnd/brontide"
 	"github.com/lightningnetwork/lnd/chanacceptor"
 	"github.com/lightningnetwork/lnd/chanbackup"
+	"github.com/lightningnetwork/lnd/chanfitness"
 	"github.com/lightningnetwork/lnd/channeldb"
 	"github.com/lightningnetwork/lnd/channelnotifier"
 	"github.com/lightningnetwork/lnd/contractcourt"
@@ -243,6 +244,10 @@ type server struct {
 	// channelNotifier to be notified of newly opened and closed channels.
 	chanSubSwapper *chanbackup.SubSwapper
 
+	// chanEventStore tracks the behaviour of channels and their remote peers to
+	// provide insights into their health and performance.
+	chanEventStore *chanfitness.ChannelEventStore
+
 	quit chan struct{}
 
 	wg sync.WaitGroup
@@ -1113,6 +1118,13 @@ func newServer(listenAddrs []net.Addr, chanDB *channeldb.DB,
 	// to peer online and offline events.
 	s.peerNotifier = peernotifier.New()
 
+	// Create a channel event store which monitors all open channels.
+	s.chanEventStore = chanfitness.NewChannelEventStore(&chanfitness.Config{
+		SubscribeChannelEvents: s.channelNotifier.SubscribeChannelEvents,
+		SubscribePeerEvents:    s.peerNotifier.SubscribePeerEvents,
+		GetOpenChannels:        s.chanDB.FetchAllOpenChannels,
+	})
+
 	if cfg.WtClient.Active {
 		policy := wtpolicy.DefaultPolicy()
 
@@ -1270,6 +1282,11 @@ func (s *server) Start() error {
 			return
 		}
 
+		if err := s.chanEventStore.Start(); err != nil {
+			startErr = err
+			return
+		}
+
 		// Before we start the connMgr, we'll check to see if we have
 		// any backups to recover. We do this now as we want to ensure
 		// that have all the information we need to handle channel
@@ -1385,6 +1402,7 @@ func (s *server) Stop() error {
 		s.invoices.Stop()
 		s.fundingMgr.Stop()
 		s.chanSubSwapper.Stop()
+		s.chanEventStore.Stop()
 
 		// Disconnect from each active peers to ensure that
 		// peerTerminationWatchers signal completion to each peer.