Aura Consensus Hook should enforce the relay-parent slot is equal to the Aura slot

[rphmeier]

#2551 introduced a ConsensusHook for Aura based on #2501 .

We require for Aura that the slot for parachains is set based on the slot of the relay-parent, as it's the only reliable indication of time we have access to within parachain runtimes. These checks are currently done through the parachain_system::CheckInherents trait that is evaluated during validate_block. Moving this check to the consensus hook would make it run during all block executions - both during validate_block and vanilla execute_block as used by parachain nodes. It is also a refactoring to make the logic more clear.

[slumber]

#2658