We need to follow some rules when adding new output streams.

1. New output stream need to be aligned with the
existing stream which has the earliest fetch streams.
http://crrev.com/c/360880
http://crrev.com/c/1143165

2. New output stream can not be mixed before its first fetching.
http://crrev.com/c/1341708
(It is also the CL to add pending stream.)

Now we have two methods to handle these issues.

1. Add pending streams.

Disadvantage:
    > In each function we traversing streams, we also need to consider pending streams.



2. Add new flag in dev_stream structure.

Disadvantage:
    > Need to modify buffer_shared, too. It might cause other problems.


I also copy content above to this doc. Feel free to edit it if you have any idea.
https://docs.google.com/document/d/1RAJ9FnNIEqMrviY-OoHnBh0kxhl31Rmf8FbAWi4fG5Y/edit?usp=sharing