GuldenD witness index

[blieb]

At the moment it takes a lot time to request witnessinfo of old blocks. I can take more than 60s. Maybe a good option to add option for indexing the witnessinfo for blocks like the txindex option for transactions.

# time Gulden-cli getwitnessinfo 295af50cfd7c98f4f2d10d3097821ed5648f686a4a6e6bf5e05351e02e220f81
[
  {
    "pow2_phase": 2,
    "number_of_witnesses_raw": 16,
    "number_of_witnesses_total": 16,
    "number_of_witnesses_eligible": 0,
    "total_witness_weight_raw": 4369988,
    "total_witness_weight_eligible_raw": 0,
    "total_witness_weight_eligible_adjusted": 0,
    "selected_witness_address": ""
  }
]

real    0m32.369s
user    0m0.000s
sys    0m0.004s