[blob] Add logs to cleanup task

Summary:
Added logs to cleanup task to help debugging. I didn't add them previously because I didn't want to bloat the (already a bit complicated) diffs with logic.

Depends on D9413

Test Plan: RUST_LOG=blob=trace cargo run -- cleanup displayed the logs.

Reviewers: michal, varun, jon

Reviewed By: michal

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9414