Fix double invocation of schedule hooks for delayed jobs

[top-sigrid]

Fixes #810

Summary

• Move invoking schedule hooks from enqueue_from_config to enqueue_recurring
• This ensures hooks are called for cron jobs but not again when delayed jobs are transferred to the work queue
• Extract extract_klass_and_params helper to avoid duplication
• Update README to clarify when hooks are called

[top-sigrid]

Regarding my discussion points from #810 I went with the solution that for enqueing recurring cron jobs the schedule-hooks are also called. Altgough that doesn't mean they are "placed on the delayed queue" I think this actually is a scheduling operation. Also #792 just a year ago quite explicitely introduced this. Hence I adjusted the Readme on this new functioning of the hooks.

I hope you are fine with this approach, cc @PatrickTulskie.

I think the CI failures are all unrelated to this fix. There is 1 actual test failure for Ruby 2.4 that I'll look into.

Let me know what you think of this and happy holidays

Edit: I also opened #813 for the flaky test (as it is unrelated to the changes in this PR)