code: func (s *Server) forgetConn(c net.Conn) {
  	if _, ok := s.conns[c]; ok {
  		delete(s.conns, c)
  		s.wg.Done()
  	}
  }
docs: // forgetConn removes c from the set of tracked conns and decrements it from the
  // waitgroup, unless it was previously removed.
  // s.mu must be held.