Distributed partitioned tables improvement areas [parent issues]

We implemented partitioning on distributed table with Postgres 10. With PG 11+, there has been several improvements/changes on Postgres that we have partially reflected in Citus. And, today we keep getting issues around integration of these new features. We also had other bugs/not implemented features for partitioning.

Some of the issues I can see on github (potentially more):

- [ ] https://github.com/citusdata/citus/issues/2064 (sql bug)
- [x] https://github.com/citusdata/citus/issues/4483 (sql bug)
- [x] https://github.com/citusdata/citus/issues/4979 (sql bug)
- [ ] https://github.com/citusdata/citus/issues/4962 (sql bug)
- [ ] https://github.com/citusdata/citus/issues/4845 (sql bug)
- [ ] https://github.com/citusdata/citus-enterprise/issues/284 (rebalancer bug)
- [ ] https://github.com/citusdata/citus/issues/4949 (rebalancer bug)
- [x] https://github.com/citusdata/citus-enterprise/issues/587 (performance)
- [x] https://github.com/citusdata/citus/issues/4900 (performance)
- [x] https://github.com/citusdata/citus/issues/4980 (sql limitation)
- [x] https://github.com/citusdata/citus/issues/4948(sql limitation)
- [x] https://github.com/citusdata/citus/issues/4938 (sql limitation)
- [ ] https://github.com/citusdata/citus/issues/4868 (regression tests)
- [ ] https://github.com/citusdata/citus/issues/4853 (columnar / performance)
- [ ] https://github.com/citusdata/citus/issues/4911 (multi-level partitioning)
- [x] https://github.com/citusdata/citus/issues/5138
- [ ] https://github.com/citusdata/citus/issues/5422