Open transaction for multi-db apps

With a multiple database application `rails console --sandbox` becomes
problematic. We can't rollback any modifications for every database
because we can't open transaction for the connections that haven't not
yet been loaded.

To solve this I handled that if the database is connected then it
will open transaction.

activerecord/lib/active_record/railties/console_sandbox.rb

@@ -1,7 +1,5 @@
 # frozen_string_literal: true
 
-ActiveRecord::Base.connection.begin_transaction(joinable: false)
-
-at_exit do
-  ActiveRecord::Base.connection.rollback_transaction
+ActiveRecord::ConnectionAdapters::AbstractAdapter.set_callback(:checkout, :after) do
+  begin_transaction(joinable: false)
 end