[bug] when broker's toppars no out buf, it will lead to delete toppar send timer
[bug] now we send a 200 status code other than 0
1. [bug] previous when metadata request is sending and the meta broker's writeable event is triggered, it will cause zero buf error. now only the metadata request is sended, than we create metadata request.
2. fix printf format error
1. fix print format error. 2. we do not delete the send timer when partition's leader is not avaliable.