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:

All but the last case are uninteresting and work like exising uploads with file.uploadhash (which we can eventually deprecate).

In the last case:

Client, file.querychunks(): Give me a list of chunks that I should upload.

This returns all the chunks for the file. Chunks have a start byte, an end byte, and a "complete" flag to indicate that the server already has the data.

Then, the client fills in chunks by sending them:

Client, file.uploadchunk(): Here is the data for one chunk.

This stuff doesn't work yet or has some caveats:

• I haven't tested resume much.
• Files need an "isPartial()" flag for partial uploads, and the UI needs to respect it.
• The JS client needs to become chunk-aware.
• Chunk size is set crazy low to make testing easier.
• Some debugging flags that I'll remove soon-ish.
• Downloading works, but still streams the whole file into memory.
• This storage engine is disabled by default (hardcoded as a unit test engine) because it's still sketchy.
• Need some code to remove the "isParital" flag when the last chunk is uploaded.
• Maybe do checksumming on chunks.

Test Plan:

• Hacked up arc upload (see next diff) to be chunk-aware and uploaded a readme in 18 32-byte chunks. Then downloaded it. Got the same file back that I uploaded.
• File UI now shows some basic chunk info for chunked files:

{F336434}

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: joshuaspence, epriestley

Maniphest Tasks: T7149

Differential Revision: https://secure.phabricator.com/D12060