[blob] Implement cleanup helper functions

Summary:
Implementation of helpers introduced in D9350. These are part of the cleanup logic
and are extensively used in the next diff

Depends on D9350

Test Plan: The code compiles, these are tested thoroughly in the next diff

Reviewers: tomek, michal, varun, jon

Reviewed By: tomek, michal

Subscribers: ashoat, wyilio

Differential Revision: https://phab.comm.dev/D9351