[lib] Memoize userinfos in usePotentialMemberItems

Summary: This addresses the urgent issue outlined in ENG-6139. I should've memoized the userInfos object.

Test Plan: Since usePotentialMemberItems is used in both chat-thread-composer.react.js and add-members-modal.react.js (both also mentioned in the original bug report), I made sure that those both no longer completely freeze/crash.

Reviewers: ashoat, michal

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D10409