Feature parity between PostgreSQL and SQLite backends

[danielhollas]

This is a tracking meta issue for collecting known functional differences between the two DB backends.

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1. 🐛 Strange behavior for !has_key filter with SQLite storage backends #6256
   topic/query-builder topic/storage type/bug +3
2. SqliteDosStorage: Make the migrator compatible with SQLite #6429
   +0
3. [devops] Run tests with sqlite backend #6425
   +0
   
4. test_delete_through_backend fails with SQLite: sqlalchemy.orm.exc.ObjectDeletedError #6436
   priority/important topic/database-backend type/bug +3
5. QueryBuilder: 'contains' operator not implemented for SQLite storage backends #6494
   topic/database-backend topic/storage type/feature request +3
   
6. QueryBuilder: implement the has_key operator properly for SQLite based storage plugins #6498
   topic/query-builder type/feature request +2
7. Reimplement has_key for SQLite #6552
   topic/storage type/feature request +2
   
8. Test feature parity between PSQL and SQLite DB backends #6621
   topic/database-backend topic/storage +2