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

Existing code: Fetches the default_role when constructed threadInfos to set the isDefault flag for each role in threadInfo.roles

Depends on D9979

Addresses part of ENG-5833

Test Plan:
Updated my query and logged the threadInfo.roles. Confirmed that for each role, isDefault was set to true for all Members and false otherwise.

Another test I did was to compare the logged threadInfo.roles both before and after the query change and they were identical

Reviewers: atul, ginsu, tomek

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D9980