CouchDB provider: if data is binary and large, store as a CouchDB attachment

[DRK3]

CouchDB has something called "attachments" that could be used to store binary data in a more efficient form. We should make it so that if the binary data is greater than some size, then it gets stored as an attachment. When retrieving documents from CouchDB, we must then check to see if they have an attachment. If they do, then we return the attachment data. Otherwise, we look at the usual "value" field.