[CSL-30] Refactoring, return ProcessBlockRes from startAltChain
[CSL-30] Refactoring, return ProcessBlockRes from startAltChain

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