func (s *gossipSender) Flush() bool {
	ch := make(chan bool)
	s.flush <- ch
	return <-ch
}
func (s *gossipSender) Flush() bool {
	ch := make(chan bool)
	s.flush <- ch
	return <-ch
}
// Flush sends all pending data, and returns true if anything was sent since
// the previous flush. For testing.