Add also the entity UUID in rdf_sync_uri table

Problem/Motivation

Sometimes you'll need to get a list of entity UUIDs given a list of URIs. This needs a round trip involving an entity query and entity load(Multiple). Too expensive given that we can store also the UUID. BTW, the minimal columns are entity_type, entity_id, uri. The bundle column was added also for convenience, to allow quick filtering without needing an additional entity query.

Proposed resolution

Add a new, unique, uuid column in rdf_sync_uri table to store the entity URI.

Remaining tasks

User interface changes

API changes

Data model changes