Upload file in chunks

[maskaravivek]

Summary:

We are going to add support for pause/resume of downloads as part of #746.

Pause/resume can be achieved by using stashed uploads. So as a first step, I am planning to add change our upload code so that any file upload is processed in chunks.

Once we have this working, it would be easy to build on top of it to add the ability to pause/resume downloads.