[lib] Discard keyserver usernames for incremental state sync and processUpdatesActionType

Summary: issue: ENG-8365

Test Plan: Tested that if incremental state sync (or process updates) is dispatched, then for new user infos their username is set to null and for old user infos (that the clients already has in their store) username is unchanged.

Reviewers: tomek, kamil

Reviewed By: kamil

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D12512