Refactor Rails ActiveRecord to use standalone ActiveRecord

[delner]

This pull request does a few major refactors:

• Moves ActiveRecord functions in Rails::Utils to ActiveRecord::Utils
• Removes ActiveRecord from the Rails integration entirely; Rails instead configures an ActiveRecord integration (just like Rack.)
• Changes ActiveRecord integration to use the ActiveSupport::Notifications subscription module.

Breaking changes:

• Tag rails.db.vendor becomes active_record.db.vendor
• Tag rails.db.name becomes active_record.db.name
• Tag rails.db.cached becomes active_record.db.cached
• Instantiation spans' service name now defaults to active_record (instead of the Rails service name) if they don't belong to a parent. Can still be overridden always with orm_service_name.

Overall, no applications should require reconfiguration.

[palazzem]

I would prefer to not include this change otherwise users with default settings will get their services renamed. Let's keep the current value that is mysql2.