[CSL-671] 'getSlotStart' implemented
[CSL-671] 'getSlotStart' implemented

input-output-hk/pos-haskell-prototype,input-output-hk/pos-haskell-prototype,input-output-hk/pos-haskell-prototype,input-output-hk/cardano-sl,input-output-hk/cardano-sl,input-output-hk/cardano-sl,input-output-hk/cardano-sl