Enable CouchDB for CSS large object storage

[dabooz]

As of writing this issue, MongoDB is the only production capable database that can be configured for cloud side storage. Mongo recently changed their licensing terms, making it harder to consume the open-horizon platform into a commercial product. This issue will extend the sync service to support CouchDB as an alternative.

The storage component of the sync-service has a golang interface that is used by the core to persist object metadata and the objects themselves. The task is to provide an implementation of that interface which works with CouchDB.