Implement a configurable thread pool for JSONRPC requests

[sgerbino]

Resolves #42

Brief description

Adds a configurable thread pool to handle JSONRPC requests with.

Checklist

• [x] I have built this pull request locally
• [x] I have ran the unit tests locally
• [x] I have manually tested this pull request
• [x] I have reviewed my pull request
• [x] I have added any relevant tests

Demonstration

❯ curl -d '{"method":"chain.get_resource_limits", "jsonrpc":"2.0", "id":1, "params":{}}' http://localhost:8080/ | jq
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100   324  100   248  100    76  16442   5038 --:--:-- --:--:-- --:--:-- 46285
{
  "jsonrpc": "2.0",
  "result": {
    "resource_limit_data": {
      "disk_storage_limit": "204800",
      "disk_storage_cost": "164",
      "network_bandwidth_limit": "1048576",
      "network_bandwidth_cost": "25",
      "compute_bandwidth_limit": "287500000",
      "compute_bandwidth_cost": "1"
    }
  },
  "id": 1
}