code: public List<WebSocketConnection> get(String key) {
          final List<WebSocketConnection> retList = new ArrayList<>();
          accept(key, C.F.addTo(retList));
          return retList;
      }
docs: Return a list of websocket connection by key
  
  @param key
  the key to find the websocket connection list
  @return a list of websocket connection or an empty list if no websocket connection found by key