Adjust discv5_updateNodeInfo JSON-RPC to specify key and value

[kdeme]

Accompanying spec PR: ethereum/portal-network-specs#207

The current version was not following the spec, but also not the above spec PR, as a nameless tuple basically required the parameter to look like:

{
  "params": [
    [
      {
        "Field0": "0xabcd",
        "Field1": "0xffff"
      },
      {
        "Field0": "0xffff",
        "Field1": "0xffff"
      }
    ]
  ]
}

Which now becomes:

{
  "params": [
    [
      {
        "key": "0xabcd",
        "value": "0xffff"
      },
      {
        "key": "0xffff",
        "value": "0xffff"
      }
    ]
  ]
}

I don't think there is some way to define a Nim type that results in something like:

{
  "params": [
    [
      {
        "0xabcd": "0xffff"
      },
      {
        "0xffff": "0xffff"
      }
    ]
  ]
}

I think that requires custom parsing of the JsonNode, so going with the more verbose version.