Clean up & Optimize session_store config

See this code block

The Rails::Application::Configuration#session_store method has several issues. First of all, when trying to set this up with DataMapper, I assumed that session_store(:data_mapper_store) would be the way to enable DataMapper session storage. This does not work, however, because DataMapper follows ActiveRecord's session class structure (Rails::DataMapper::SessionStore, just like Rails::ActiveRecord::SessionStore).

Rather than following an agnostic approach that all ORMs can adopt, ActiveRecord session support is hardcoded into this method.

In order to make this work with session_store(:data_mapper_store), the DataMapper team would have to rename or alias the existing class to ActionDispatch::Session::DataMapperStore. However, even if they make that change, this method is caching the symbol as @session_store and performing the string conversion/constant lookup every time it is called. Even if the structure of this method is not changed, we should be caching the Class itself once we've looked it up.

One other issue at this point is that I can configure my session store with MyApp::Application.config.session_store = Rails::DataMapper::SessionStore, but even if I do that I still have to manually require 'dm-rails/session_store' before declaring my session_store. It would be great if a solution could also handle autoloading of the required class.

I'd like some advice on how this should be resolved. It seems reasonable that AR should not be hardcoded into this method, but should instead follow the same agnostic approach that every other ORM should follow. Exactly what that approach should be is open for debate.