[server][hapi][json] Support receiving event

JSON must be the following format:

    {
      "type": "event",
      "body": {
        {
          "id": <Event ID as integer>,
          "timestamp": <UNIX time as float>,
          "hostName": <Host name as string>,
          "message": <Log message as string>
        }
      }
    }

The format may be changed in the future. It is an experimental
specification.

TODO:

  * Extract JSON message processing code (such as getting and
    validating) as another class.
Are you intensinally avoid using JSONParserAgent?