#9077
Where possible replaces all instances of `validator_monitor::timestamp_now` with `chain.slot_clock.now_duration().unwrap_or_default()`.
Where chain/slot_clock is not available, instead replace it with a convenience function `slot_clock::timestamp_now`.
Remove the `validator_monitor::timestamp_now` function.
Co-Authored-By: Mac L <mjladson@pm.me>