[keyserver] new fetchPrivilegedThreadInfos function

Summary:
unlike fetchThreadInfos, this function does not filter out threads that are inaccessible to the given user

Depends on D13851

we need this function to get all the threadInfos for community root threads. we'll use this data on clients to populate a directory of communities for users to join

Test Plan: successfully got back threadInfos for all threads when calling responder in next diff

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13852