- ### Problem

  Connection pools are not properly shared. A replica can't see the
  data in the open transaction on the writing connection.
  ```ruby
  Dog.create!(name: 'bilou')

  AnimalsBase.connected_to(role: :reading) do
    Dog.find_by(name: 'bilou') # No result
  end
  ```

  The reason of this bug is because when test starts, we iterate
  over `AR::Base.connection_handlers` which only contains the `writing`
  handler since models haven't been loaded (app isn't eagerloaded
  test env).

  ### Solution

  Share connection pools as soon as a connection is established
  by creating a subcriber.