[keyserver] Determine which entities to fetch in a spec

Summary:
Add a function to a spec which determines which threads, entries, etc. need to be fetched.

Depends on D9653

Test Plan: Generate some updates, reintroduce old code, and compare new with old result using _.isEqual

Reviewers: kamil, inka, bartek

Reviewed By: kamil

Subscribers: ashoat, wyilio

Differential Revision: https://phab.comm.dev/D9654