code: public ZMsg command(Command command, ZMsg msg, boolean sync)
      {
          if (command == Command.CONFIG) {
              return configure(msg);
          }
          if (command == Command.RESTART) {
              String status = restart(msg);
              msg = new ZMsg();
              msg.add(status);
              return msg;
          }
          return null;
      }
docs: Sends a command message to the proxy actor.
  Can be useful for programmatic interfaces.
  Works only with commands {@link Command#CONFIG CONFIG} and {@link Command#RESTART RESTART}.
  
  @param command  the command to execute.
  @param msg      the custom message to transmit.
  @param sync     true to read the status in synchronous way, false for asynchronous mode
  @return the response message