[lib] Drill allAtOnce from useChatMentionCandidatesObjAndUtils to useENSNames

Summary:
This solves ENG-5274. Additional context about allAtOnce in D9515.

Depends on D9466

Test Plan:
I compared performance of the following two scenarios:

1. master + revert of D9465 + D9466
2. master + revert of D9465 + D9466 + this diff

While I did perceive a slight regression in TTI on the inbox by about a second, I think it's okay to land this stack (including @tomek's work) after this diff is accepted.

More details on next steps here.

Reviewers: tomek, atul

Reviewed By: atul

Subscribers: wyilio, tomek

Differential Revision: https://phab.comm.dev/D9523