fix MQTT transfer bug && use short product_id

[fix]:
1.  get recv msg topic and pass to user to reply to this topic, but MQTT
topic data is not null-terminated, so topic length is needed to pass to
user(easyCloudMsgRecvCallback) too.

[update]
1.  MQTT message(send/recv) payload max size to 512 bytes;
2.  use new short product_id like  "product/xxxx".
version to v0.2.4