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