routing: perform path finding inside a single DB transaction

This commit modifies the path finding logic such that all path finding
is done inside a _single_ database transaction. With this change, we
ensure that we don’t end up possibly creating hundreds of database
transactions slowing down the path finding and payment sending process
all together.

routing/router.go

@@ -978,14 +978,24 @@ func (r *ChannelRouter) FindRoutes(target *btcec.PublicKey,
 		return nil, err
 	}
 
+	tx, err := r.cfg.Graph.Database().Begin(false)
+	if err != nil {
+		tx.Rollback()
+		return nil, err
+	}
+
 	// Now that we know the destination is reachable within the graph,
 	// we'll execute our KSP algorithm to find the k-shortest paths from
 	// our source to the destination.
-	shortestPaths, err := findPaths(r.cfg.Graph, r.selfNode, target, amt)
+	shortestPaths, err := findPaths(tx, r.cfg.Graph, r.selfNode, target,
+		amt)
 	if err != nil {
+		tx.Rollback()
 		return nil, err
 	}
 
+	tx.Rollback()
+
 	// Now that we have a set of paths, we'll need to turn them into
 	// *routes* by computing the required time-lock and fee information for
 	// each path. During this process, some paths may be discarded if they
@@ -1032,7 +1042,7 @@ func (r *ChannelRouter) FindRoutes(target *btcec.PublicKey,
 		return validRoutes[i].TotalFees < validRoutes[j].TotalFees
 	})
 
-	log.Tracef("Obtained %v paths sending %v to %x: %v", len(validRoutes),
+	go log.Tracef("Obtained %v paths sending %v to %x: %v", len(validRoutes),
 		amt, dest, newLogClosure(func() string {
 			return spew.Sdump(validRoutes)
 		}),