Fix decorrelation of sub-queries.

- Update planner to do decorrelation.
- Fixes to decorrelation logic.
- Update PushFilterPastJoinRule to call planner's copy hook, and use JoinRelBase instead of JoinRel.
- Add filter push down on decorrelation.
- Fix RelStructuredTypeFlattener to correctly populate toRelContext.
- Code formatting. (Julian Hyde)
- Add test based upon TpchSchema.