Adjust the constants for benchmarking
Adjust the constants for benchmarking

- minimal fees are set to zero, since otherwise the transactions from
  the generator will be invalid (they are balanced)
- increased k to 6 -- with k=2, it was too easy to go into recovery
  mode, or to end up with an unacceptable chain quality end stop the
  system.

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