Add indexes for the RemoteMirror.stuck query

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Currently we're using this query to mark all these mirrors as failed.

The query is always scoped to a project, so they're never more than a handful of rows to update.

But Ideally we'd have an index for project_id, last_update_started_at, last_update_at after https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31247 gets merged.