R1:4aed453b06e4

Add a chunking storage engine for files

Summary:
Ref T7149. This isn't complete and isn't active yet, but does basically work. I'll shore it up in the next few diffs.

The new workflow goes like this:

> Client, file.allocate(): I'd like to upload a file with length L, metadata M, and hash H.

Then the server returns `upload` (a boolean) and `filePHID` (a PHID). These mean:

| upload | filePHID | means |
|---|---|---|
| false | false | Server can't accept file.
| false | true | File data already known…