Idempotent DELETE by ids, optimizing performance

[andrus]

Currently during DELETE, if the object corresponding to a specified ID is not found, a 404 exception is thrown. If we run multiple IDs, any one object that is not found will block the entire operation, which goes against the idempotent nature of DELETE. So the changes per this task:

• If DELETE has multiple IDs, all objects are fetched at once (this is a nice performance optimization)
• If some objects are not matched, all other objects should still be deleted
• The response will be 200 in all cases, except one - when zero matching objects where found. In that case Agrest will continue returning 404