[blob] Add struct for representing unchecked blobs

Summary:
Added a struct for representing unchecked items in database. It greatly simplifies the cleanup job.
It is used in the next two diffs

Depends on D9349

Test Plan: Code compiles, implementation is tested in the next two diffs.

Reviewers: tomek, michal, varun, jon

Reviewed By: tomek, michal

Subscribers: ashoat, wyilio

Differential Revision: https://phab.comm.dev/D9350