[keyserver] Generalize state_check status handling using specs

Summary:
Instead of computing hash for each store by hand, iterate over specs and compute all the hashes for them.

https://linear.app/comm/issue/ENG-4631/migrate-checkstate-from-session-utils-to-a-spec

Depends on D8946

Test Plan: Bring back the old hash logic and compare the results with the new version.

Reviewers: kamil, bartek, inka

Reviewed By: bartek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D8948