Fixes #2955: Changed the signature of ActiveQuery constructors and `ActiveRecord::createQuery()` to simplify customizing ActiveQuery classes

Fixes #2726: ActiveRecord now fills default values on creating new instance of the model if defaults are available from DB schema

close #2543

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

- DbMessageSource now performs UNION ALL.
- Fixed message merging for php and po.
- Updated tests.

fixes #1993

meet the AR interface requirements.

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