copy fron http://code.taobao.org/p/myrelay/wiki/index/， and do some special changes
read client query (cli_query_cb(), change conn.state = STATE_WRITING_MYSQL) | V send query to mysql (my_query_cb(), change conn.state = STATE_READ_MYSQL_WRITE_CLIENT) | V read mysql result and send to client (cli_query_cb : my_answer_cb)