routing: store missioncontrol state in blocks and eliminate cursor use

This commit changes missioncontrol's store update from per payment to
every second. Updating the missioncontrol store on every payment caused
gradual slowdown when using etcd.
We also completely eliminate the use of the cursor, further reducing
the performance bottleneck.

server.go

@@ -746,6 +746,7 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 		&routing.MissionControlConfig{
 			ProbabilityEstimatorCfg: estimatorCfg,
 			MaxMcHistory:            routingConfig.MaxMcHistory,
+			McFlushInterval:         routingConfig.McFlushInterval,
 			MinFailureRelaxInterval: routing.DefaultMinFailureRelaxInterval,
 		},
 	)
@@ -1663,6 +1664,12 @@ func (s *server) Start() error {
 			return nil
 		})
 
+		s.missionControl.RunStoreTicker()
+		cleanup.add(func() error {
+			s.missionControl.StopStoreTicker()
+			return nil
+		})
+
 		// 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
@@ -1869,6 +1876,7 @@ func (s *server) Stop() error {
 			srvrLog.Warnf("failed to stop chanSubSwapper: %v", err)
 		}
 		s.chanEventStore.Stop()
+		s.missionControl.StopStoreTicker()
 
 		// Disconnect from each active peers to ensure that
 		// peerTerminationWatchers signal completion to each peer.