[keyserver] Update ServerThreadInfo to only cover ThinThreadType

Summary:
ServerThreadInfos are only used on the keyserver, and as such they don't include thick threads.

This diff is necessary to make sure that the changes in the next don't cause Flow errors.

Depends on D12656

Test Plan: Make sure the next diff doesn't have any Flow errors

Reviewers: tomek, marcin, inka

Reviewed By: inka

Differential Revision: https://phab.comm.dev/D12660