allow extending only one class to add scopes, fixes #2146

TODO:

- [ ] adjust guide docs
- [ ] adjust README files of extensions
- [ ] finish work and fix test breaks

- refactored elasticsearch AR to make type and index available in
  instantiate(), issue #1313

fixes #2281

fixes #1864

fixes #1993

meet the AR interface requirements.

now supports mapped primary key when _id is part of source with an
alias.