DB backend API: add documentation & improvements

- [x] remove obsolete arguments to db.compile_selectors()
- [x] think about, if we can simplify resolving set statements / classes
- [ ] simplify and standardize parameters to objects_* functions
- [ ] make objects_* functions ready for bulk processing (e.g. get parents of several objects at 

These improvements might already be integrated into the documentation as it is written in this pull request, but only implemented when they are checked.