routing: implement 2-week zombie channel pruning

This commit implements 2-week zombie channel pruning. This means that
every GraphPruneInterval (currently set to one hour), we’ll scan the
channel graph, marking any channels which haven’t had *both* edges
updated in 2 weeks as a “zombie”. During the second pass, all “zombie”
channel are removed from the channel graph all together.

Adding this functionality means we’ll ensure that we maintain a
“healthy” network view, which will cut down on the number of failed
HTLC routing attempts, and also reflect an active portion of the graph.

server.go

@@ -266,6 +266,8 @@ func newServer(listenAddrs []string, chanDB *channeldb.DB, cc *chainControl,
 
 			return s.htlcSwitch.SendHTLC(firstHopPub, htlcAdd, errorDecryptor)
 		},
+		ChannelPruneExpiry: time.Duration(time.Hour * 24 * 14),
+		GraphPruneInterval: time.Duration(time.Hour),
 	})
 	if err != nil {
 		return nil, fmt.Errorf("can't create router: %v", err)