Update /fee_stats response

[fracek]

• ✂ Remove the following fields:

  min_accepted_fee
  mode_accepted_fee
  p10_accepted_fee
  p20_accepted_fee
  p30_accepted_fee
  p40_accepted_fee
  p50_accepted_fee
  p60_accepted_fee
  p70_accepted_fee
  p80_accepted_fee
  p90_accepted_fee
  p95_accepted_fee
  p99_accepted_fee
  

• ➕Add support for max_fee and fee_charged fields. Each field contains a JSON object that looks like this:

  {
    "last_ledger": "22606298",
    "last_ledger_base_fee": "100",
    "ledger_capacity_usage": "0.97",
    "fee_charged": {
      "max": "100",
      "min": "100",
      "mode": "100",
      "p10": "100",
      "p20": "100",
      "p30": "100",
      "p40": "100",
      "p50": "100",
      "p60": "100",
      "p70": "100",
      "p80": "100",
      "p90": "100",
      "p95": "100",
      "p99": "100"
    },
    "max_fee": {
      "max": "100000",
      "min": "100",
      "mode": "100",
      "p10": "100",
      "p20": "100",
      "p30": "100",
      "p40": "100",
      "p50": "100",
      "p60": "100",
      "p70": "100",
      "p80": "100",
      "p90": "15000",
      "p95": "100000",
      "p99": "100000"
    }
   }
  

  See Update /fee_stats documentation. stellar/go#2140 for more info.

[Kirbyrawr]

#215