[CSM-377] Don't use `downloadHeader` for `localChainDifficulty`
[CSM-377] Don't use `downloadHeader` for `localChainDifficulty`

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