Files now share storage. New files are written only if no other files have the same contentHash. Storage is only deleted if no other file is sharing it.

Summary: Storage is shared between files in a smart way. When uploading files, if other file have the same contentHash, then share storage. On delete, storage is permanently deleted only if no other files are sharing it

Test Plan: Upload multiple copies of the same file, while tracking database. Delete copies of files and check to see that the storage is only deleted if no other files are using it

Reviewers: epriestley

CC: aran, Korvin

Maniphest Tasks: T2454

Differential Revision: https://secure.phabricator.com/D4775