Refactor: extract most aura logic out to standalone module, make use of these (#13764)

* Extract most aura logic out to standalone module, make use of these

* Update client/consensus/aura/src/standalone.rs

improve docs

Co-authored-by: Bastian Köcher <git@kchr.de>

* add slot_duration_at

* ".git/.scripts/commands/fmt/fmt.sh"

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Co-authored-by: Bastian Köcher <git@kchr.de>
Co-authored-by: parity-processbot <>