Fix Rails autoloading

[p8]

Delay initializing of backend until all initializers have run when using Rails.

As mentioned in #106 (comment) initializing ActiveRecord after this gem is loaded prevents problems with Rails.application.config.active_record.belongs_to_required_by_default.
But instead of using on_load(:active_record) like in #106 this use the on_load(:after_initialize) hook. on_load(:after_initialize) always runs but after Rails initializes so it won't cause problems with rake tasks.
See: https://guides.rubyonrails.org/engines.html#configuration-hooks

require "delayed/backend/active_record" has been removed since it's already called in Delayed::Worker.Backend=

[p8]

The build seems to fail for unrelated reasons.

[p8]

I've rebased against #173 to get the build green.

[p8]

@albus522 I've been running this patch on some production apps without any problems. Anything I can do to get this merged?

[albus522]

Can you hunt down when ActiveSupport.on_load(:after_initialize) {} started being preferred over config.after_initialize {} to make sure we don't need a compatibility layer? We still support all the way back to ActiveRecord 3.0 as we haven't had a need to break that support.

[p8]

Ok, so config.after_initialize {} has been supported for almost 10 years.
https://github.com/rails/rails/blame/98a57aa5f610bc66af31af409c72173cdeeb3c9e/railties/lib/rails/railtie/configuration.rb#L70
And it's still the recommended usage: https://api.rubyonrails.org/classes/Rails/Railtie.html (see Configuration)
I'll change the pull request to use config.after_initialize {} with a Railtie instead.

[p8]

@albus522 I've pulled in #106 and added a commit to use config.after_initialize {}

[p8]

The build fails for unrelated reasons.