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.

routing/router_test.go

@@ -84,6 +84,8 @@ func createTestCtx(startingHeight uint32, testGraph ...string) (*testCtx, func()
 			_ *lnwire.UpdateAddHTLC, _ *sphinx.Circuit) ([32]byte, error) {
 			return [32]byte{}, nil
 		},
+		ChannelPruneExpiry: time.Hour * 24,
+		GraphPruneInterval: time.Hour * 2,
 	})
 	if err != nil {
 		return nil, nil, fmt.Errorf("unable to create router %v", err)