Refactor update-cache

[Macroz]

The rems.db.applications/update-cache function has some room for later refactoring, as it is getting pretty big. maybe we could split it up somehow? one idea would be

[apps change-set] (calculate-updated-apps state ...)
[users change-set] (calculate-updated-users state change-set ...) ; merge change-sets
[roles change-set] (calculate-updated-roles state change-set ...)
new-state (merge apps users roles) ; or (select-keys apps [::raw-apps ::enriched-apps]) etc.

Originally posted by @aatkin in #3305 (comment)