Swift Server-side Encryption through a proxy middleware

Provide encryption for data-at-rest:
- containers are marked as encrypting upon creation (immutable)
- use per-object key which is wrapped with a higher key, either in a simple hierarchy (one master key wrapping all object keys) or full direct hierarchy (acc-master -> account -> contianer -> object)
- master key to reside in a key manager, e.g. Barbican
- the hierarchy, key manager, and, possibly, the crypto library to be made pluggable and set in proxy-server.conf
- support efficient partial reads