Use sendTargetBacklog when re-attaching a channel

No need to attempt to send backlog for all targets in the network.
We're only interested in a single channel.

user.go

@@ -226,9 +226,7 @@ func (net *network) attach(ch *Channel) {
 			forwardChannel(dc, uch)
 		}
 
-		if _, ok := net.delivered[ch.Name]; ok {
+		dc.sendTargetBacklog(net, ch.Name)
-			dc.sendNetworkBacklog(net)
-		}
 	})
 }