[lib] only attempt to fetch avatars from auth keyserver once per session

Summary:
Resolves https://linear.app/comm/issue/ENG-9065/make-sure-userinfoshandler-doesnt-query-authoritative-keyserver

We want to make sure UserInfosHandler doesn't query for a single user's avatar from the keyserver more than once per app start. We accomplish this by maintaining a set of user IDs for which we've requested avatars

Test Plan: Logged the contents of userIDsWithoutOwnID without this change and saw the same user IDs appearing repeatedly. After this change, user IDs only appeared once in the logs

Reviewers: ashoat, inka

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13235