[keyserver] Fix memberOfContainingThreadPromise behavior for users missing in DB

Summary:
If a containing thread exists, then everybody should be either a member or non-member. If no row is found for them in the DB, they should be considered a non-member.

This solves ENG-7384. More details there.

Test Plan: Confirm that the thread creation now fully errors out, and returns an invalid_credentials ServerError

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D11377