[blob] Use new blob-exists check in single endpoint

Summary:
Despite D13481, multiple GetItem DDB calls were still happening inside service.assign_holder(). Removed that call there and updated single endpoint to use the new way.

Updated single assign-holder endpoint to use the new batch function.

Depends on D13481

Test Plan:

• Made sure data_exists has still correct value for the POST /blob endpoint
• Blob integration tests

Reviewers: will, varun, kamil, marcin

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13482