babe: don't repeatedly lookup keys in authorship rpc function

Expose a new function `claim_slot_using_keypars` in Babe so that the `babe_epochAuthorship` can
lookup authorship for all slots in the epoch without repeatedly looking up keys in the keystore.

Time to run the `babe_epochAuthorship` RPC call goes from 7s to 25ms on a local dev chain on my
machine.