Improve the `IssuableFinder` when scoping issuables to a running milestone
The query for filtering issuables by milestones could be improved. This came up in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/25969
One suggestion was using an EXISTS SELECT 1
query instead of id IN (SELECT id...)
@NikolayS Suggested using explain (buffers, analyze)
to see what's going on with the current query.