Support Chunking in StreamData (#248)

Summary:

Another feature in the new chunking policy described in this [doc](https://fburl.com/gdoc/gkdwwju1) is the ability to split large streams above a specified limit into smaller chunks. In this diff, we implement a `popChunk` method in each `StreamData` class to handle this functionality. With this feature we are not forced to encode extremely large streams into a single chunk.

Integration will happen in the next diff.

Differential Revision: D81824143

Author: macvincent