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