Make multi-chain polymorphism more efficient

[benjie]

https://github.com/benjie/crystal/blob/main/postgraphile/postgraphile/__tests__/queries/polymorphic/vulns.union_owners.simple.test.graphql

1. we should be able to do a DISTINCT query so same records don't appear multiple times
2. shouldn't need so many queries
3. queries used should be able to group by target table, perhaps (would link to DISTINCT)