test: add async txn seen notifications to network harness

This commit adds a new feature to the network harness enabling callers
to receive async notifications once a particular transaction is seen on
the network. Such a feature is useful when due to the asynchronous
behavior of node communications.

With this new feature, tests can now wait for a particular transaction
to be seen within the network before proceeding.

rpcserver.go

@@ -270,6 +270,8 @@ out:
 				"ChannelPoint(%v): %v", targetChannelPoint, err)
 			return err
 		case closingUpdate := <-updateChan:
+			rpcsLog.Tracef("[closechannel] sending update: %v",
+				closingUpdate)
 			if err := updateStream.Send(closingUpdate); err != nil {
 				return err
 			}