Fixes for couchbase backend

This is some fixing for the couchbase backend.  It corrects a missing use of res.value, and removes the "blob" API entirely.

AFAICT, couchbase casid objects cannot be serialized - if you round-trip them through JSON they will be reported as "invalid" when you pass them back to the server.  Since the blob API assumes they can be serialized, it seems the simplest option is to just remove it.