R1:1773af6ada08

Enable the chunk storage engine

Summary: Ref T7149. This works now, so enable it.

Test Plan:
- Uploaded large and small files in Firefox, Safari and Chrome.
- Uploaded large files with `arc upload`.
- Stopped/resumed large files with all clients.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7149

Differential Revision: https://secure.phabricator.com/D12079