message: Send quit command when client close.
diff: @@ -189,6 +189,15 @@ static int swoole_mysql_coro_close(zval *this)
  return FAILURE;
  }
  + //send quit command
  + swString_clear(mysql_request_buffer);
  + client->cmd = SW_MYSQL_COM_QUIT;
  + bzero(mysql_request_buffer->str, 5);
  + mysql_request_buffer->str[4] = SW_MYSQL_COM_QUIT;//command
  + mysql_request_buffer->length = 5;
  + mysql_pack_length(mysql_request_buffer->length - 4, mysql_request_buffer->str);
  + SwooleG.main_reactor->write(SwooleG.main_reactor, client->fd, mysql_request_buffer->str, mysql_request_buffer->length);
  +
  zend_update_property_bool(swoole_mysql_coro_class_entry_ptr, this, ZEND_STRL("connected"), 0 TSRMLS_CC);
  SwooleG.main_reactor->del(SwooleG.main_reactor, client->fd);
  @@ -210,6 +219,8 @@ static int swoole_mysql_coro_close(zval *this)
  mysql_statement *stmt = node->data;
  if (stmt->object)
  {
  + // after connection closed, mysql stmt cache closed too
  + // so we needn't send stmt close command here like pdo.
  swoole_set_object(stmt->object, NULL);
  efree(stmt->object);
  }
  