Partitioning: Referential integrity without foreign keys

With partitioning in PG11, we can't have foreign keys referencing a partitioned table (PG12 supports this). Since we'll minimally support PG11 for a while, we'll have to deal with this and implement other strategies for foreign keys.

In addition to implementing cascading deletes in #201872 (closed), we'll have to find ways to

• check/enforce referential integrity and
• deal with referential inconsistencies.

In this issue, we discuss ways to maintain referential integrity in the absence of foreign keys referencing partitioned tables.