Remove CAS logic from file storage

[nagem]

As we plan our move to supporting Object Storage in core, removing existing CAS logic simplifies the transition to supporting both local and S3 style file storage.

• remove CAS from existing file storage
• write migration script to create unique file for each file in mongo
• before upgrades, advise users to use existing one-off CAS calculation script to view how much extra space will be required after one-to-one file in mongo -> file object in storage is complete