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.