[relDB2] equal_fields trigger 2

[luisa-beerboom]

These things are TODOs that got left over during the simple initial implementation in #3382

• Streamline the trigger code performance-wise, perhaps refactor it a bit too.
  • Decide if trigger errors should still be done on a field-by-field-basis, or if they should be aggregated and all thrown together. (See TODO in `projector/test_project)