[keyserver] Simplify fetching threads
Summary:
All the other fetchers return a collection directly without intermediate object - we can do the same for threads so that the code becomes simpler and more simplifications could be made further in the stack.
https://linear.app/comm/issue/ENG-4654/unify-interfaces-in-session-utils
Depends on D8941
Test Plan: Modify threads in the keyserver db and check if the state is correctly fixed.
Reviewers: kamil, bartek, inka
Reviewed By: bartek
Subscribers: ashoat
Differential Revision: https://phab.comm.dev/D8946