PROTOCOL: JS API FETCH STREAM for flv.js

Reference: https://github.com/ossrs/srs/pull/691#issuecomment-267323781
It seems that there is a new JS API that supports streaming reading with GET. However, it is unclear whether SRS needs to provide support for this or if standard HTTP GET is sufficient. Therefore, creating this bug to track it.

`TRANS_BY_GPT3`