docs: public void getMembers(final String group, MembersNotification callback) throws Exception {
          if(callback == null)
              return;
          // if(!isConnected()) throw new Exception ("not connected");
          synchronized(get_members_map) {
              List<MembersNotification> set=get_members_map.get(group);
              if(set == null)
                  get_members_map.put(group, set=new ArrayList<>());
              set.add(callback);
          }
          try {
              writeRequest(new GossipData(GossipType.GET_MBRS, group, null));
          }
          catch(Exception ex) {
              removeResponse(group, callback);
              throw new Exception(String.format("connection to %s broken. Could not send %s request: %s",
                                                gossipRouterAddress(), GossipType.GET_MBRS, ex));
          }
      }