func (s *Session) LiveServers() (addrs []string) {
	s.m.RLock()
	addrs = s.cluster().LiveServers()
	s.m.RUnlock()
	return addrs
}
func (s *Session) LiveServers() (addrs []string) {
	s.m.RLock()
	addrs = s.cluster().LiveServers()
	s.m.RUnlock()
	return addrs
}
// LiveServers returns a list of server addresses which are
// currently known to be alive.