R1:aa4adf3ab870

Add support for partially uploaded files

Summary:
Ref T7149. This flags allocated but incomplete files and doesn't explode when trying to download them.

Files are marked complete when the last chunk is uploaded.

I added a key on `<authorPHID, isPartial>` so we can show you a list of partially uploaded files and prompt you to resume them at some point down the road.

Test Plan: Massaged debugging settings and uploaded README.md very slowly in 32b chunks. Saw the file lose its "Partial" flag when the last…