graph/db: handle topology updates in a single location

In this commit, we ensure that any topology update is forced to go via
the `handleTopologySubscriptions` handler so that client subscriptions
and updates are handled correctly and in the correct order.

This removes a bug that could result from a client missing a
notification about a channel being closed if the client is subscribed
and shortly after, `PruneGraph` is called which would notify all
subscribed clients and possibly do so before the client subscription has
actually been persisted.

graph/db/graph.go

@@ -492,9 +492,12 @@ func (c *ChannelGraph) PruneGraph(spentOutputs []*wire.OutPoint,
 		closeSummaries := createCloseSummaries(
 			blockHeight, edges...,
 		)
-		c.notifyTopologyChange(&TopologyChange{
-			ClosedChannels: closeSummaries,
-		})
+
+		select {
+		case c.topologyUpdate <- closeSummaries:
+		case <-c.quit:
+			return nil, ErrChanGraphShuttingDown
+		}
 	}
 
 	return edges, nil