use translation model's connection pool

[ollym]

For situations where you might want to leverage Rails' multiple databases, eg.

class ApplicationRecord < ActiveRecord::Base
  self.abstract_class = true

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

class Word < ApplicationRecord
  extend Mobility
  translates :name, :meaning
end

Currently if you do this:

ApplicationRecord.connected_to(role: :reading) do
  Word.first.name
end

The word record is fetched using the reading role, but the translation is fetched using the writing role because the Translation class inherits from it's own abstract class.

This PR introduces some logic that will use by default the abstract class of the model that it is translating, and therefore reuse its connection properties.

[ollym]

@shioyama any chance of getting this merged? This fixes how it works with Rails' multiple databases. Rails internally adopts the same mechanism for habtm tables here: https://github.com/rails/rails/blob/main/activerecord/lib/active_record/associations/builder/has_and_belongs_to_many.rb#L45-L47

Thanks