refactor integrity reducer starting state to use processStoreOps

Summary:
This refactors the integrity reducer starting state to use processStoreOps. If empty state, we should generate a remove_all_integrity_thread_hashes op.

More context: https://phab.comm.dev/D11287

Depends on D11298

Test Plan: flow check

Reviewers: kamil, tomek

Reviewed By: kamil

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D11322