[blob] Batch check if blob exists

Summary:
Instead of calling GetItem for each blob (and get all its data) just to see if it exists, we can re-use db.list_existing_keys() which internally uses BatchGetItem to only get primary keys, all at once.
This improves DDB performance and reduces costs.

Updated batch-add-holders endpoint to use this.

Depends on D13480

Test Plan: Repeated test plan from D13480 confirmed it works the same way.

Reviewers: will, varun, kamil, marcin

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D13481