[keyserver] Update usage of threads.default_role in role-fetchers.js

Summary:
We need to get rid of usages of threads.default_role throughout the codebase so we can eventually drop the column.

This diff handles the usage in role-fetchers.js

Existing code: We determine whether or not a role is a default role based on r.id = t.default_role to then return in an object.

Depends on D9975

Addresses part of ENG-5833

Test Plan: Ran the query before and after in my MariaDB on my local database and saw the same results printed out

Reviewers: atul, ginsu, tomek, ashoat

Reviewed By: ashoat

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D9976