[lib] Discard keyserver usernames for full state sync

Summary: issue: ENG-8365

Test Plan: Checked that if users are not present in the store, their user infos are added without usernames. Checked that if they are present, their usernames don't change

Reviewers: tomek, kamil

Reviewed By: kamil

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D12510