[blob] Add db helpers for cleanup

Summary:
This replaces D8452 - I have reimplemented these methods using new batch helpers. These functions are needed for blob cleanup.

Depends on D9348

Test Plan:
Called these on a test dataset (containing combinations of checked and unchecked blobs and holders):

• list_existing_keys - returns the same keys as given input if they exist in db. For a list of non-existing keys, an empty Vec is returned.
• find_unchecked_items - confirmed that it returned only items that have unchecked attribute set.
• batch_mark_checked - confirmed that items previously having checked attribute, no longer have it.
• batch_delete_rows - confirmed that items are deleted from db.

Reviewers: michal, varun, jon

Reviewed By: michal

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9349