* RedundancyGroupSummary:
- Columns are same as in DependencyNodeSummary. Column `redundancy_group.state.failed` excluded because not required here.
- Don't sort by default. Sorting the summary is not necessary and even fails for PostgreSQL as it would require adding the column to the GROUP BY clause.
- Introduce new method for(), to easily get the summary query for given group id. Adjust filter. The summary is always for parent (member).