Operations: Allow updating the same entity multiple times in one batch (operations)

[martastain]

operations endpoint prevents duplicate targets tuple(entity_type, entity_id) for update and delete operations,
since entity is loaded first using SELECT FOR UPDATE.

#74

To lift this limit, we could reuse existing entity instances (keep all opened entities in a cache for the entire request lifecycle). That would require a refactoring of the endpoint.

Possible use case:

• change name folder-a ➝ tmp111
• change name folder-b ➝ folder-a
• change name tmp111 ➝ folder-b