Support multiple databases

[sharshenov]

Rails natively supports multi-database setups starting with version 6.0.

class AnimalsModel < ApplicationRecord
  self.abstract_class = true

  connects_to database: { writing: :primary, reading: :primary_replica }
end

This PR updates acts_as_dag to select the correct database when defining link and descendant models by inheriting from the nearest abstract class. This change allows it to support multi-database setups.