Fix double invocation of schedule hooks for delayed jobs

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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

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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)

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Hi @PatrickTulskie, just a friendly ping on my open PRs (#812, #813, #817). They've been sitting for a while now - is there anything I can do to help move them along? Happy to rebase onto the 5.0.0 release if that helps.

I also just opened #822 (pipelining count_all_scheduled_jobs) which fixed a real production bottleneck for us. Would be great to get that one in as well.