Parachains: Use relay chain slot for velocity measurement

[skunert]

closes #3967

Changes

We now use relay chain slots to measure velocity on chain. Previously we were storing the current parachain slot. Then in on_state_proof of the ConsensusHook we were checking how many blocks were athored in the current parachain slot. This works well when the parachain slot time and relay chain slot time is the same. With elastic scaling, we can have parachain slot times lower than that of the relay chain. In these cases we want to measure velocity in relation to the relay chain. This PR adjusts that.

Migration

This PR includes a migration. Storage item SlotInfo of pallet aura-ext is renamed to RelaySlotInfo to better reflect its new content. A migration has been added that just kills the old storage item. RelaySlotInfo will be None initially but its value will be adjusted after one new relay chain slot arrives.

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