routing: add strict pruning as new flag, default for neutrino

In this commit we add a new flag that controls if lnd will do strict
zombie pruning or not. If true, then this will cause lnd to maintain a
tighter graph as it wants both edges to always be live. We enable this by
default for neutrino as without this, it's possible that a node never
sees both edges begin disabled, so those edges are never actually pruned
from the graph.

server.go

@@ -768,6 +768,8 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 
 	s.controlTower = routing.NewControlTower(paymentControl)
 
+	strictPruning := (cfg.Bitcoin.Node == "neutrino" ||
+		cfg.Routing.StrictZombiePruning)
 	s.chanRouter, err = routing.New(routing.Config{
 		Graph:               chanGraph,
 		Chain:               cc.ChainIO,
@@ -784,6 +786,7 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 		NextPaymentID:       sequencer.NextID,
 		PathFindingConfig:   pathFindingConfig,
 		Clock:               clock.NewDefaultClock(),
+		StrictZombiePruning: strictPruning,
 	})
 	if err != nil {
 		return nil, fmt.Errorf("can't create router: %v", err)